# orm/query.py
# Copyright (C) 2005-2020 the SQLAlchemy authors and contributors
# <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

"""The Query class and support.

Defines the :class:`_query.Query` class, the central
construct used by the ORM to construct database queries.

The :class:`_query.Query` class should not be confused with the
:class:`_expression.Select` class, which defines database
SELECT operations at the SQL (non-ORM) level.  ``Query`` differs from
``Select`` in that it returns ORM-mapped objects and interacts with an
ORM session, whereas the ``Select`` construct interacts directly with the
database to return iterable result sets.

"""
from itertools import chain

from . import attributes
from . import exc as orm_exc
from . import interfaces
from . import loading
from . import persistence
from . import properties
from .base import _entity_descriptor
from .base import _generative
from .base import _is_aliased_class
from .base import _is_mapped_class
from .base import _orm_columns
from .base import InspectionAttr
from .path_registry import PathRegistry
from .util import _entity_corresponds_to
from .util import aliased
from .util import AliasedClass
from .util import join as orm_join
from .util import object_mapper
from .util import ORMAdapter
from .util import with_parent
from .. import exc as sa_exc
from .. import inspect
from .. import inspection
from .. import log
from .. import sql
from .. import util
from ..sql import expression
from ..sql import util as sql_util
from ..sql import visitors
from ..sql.base import ColumnCollection
from ..sql.expression import _interpret_as_from
from ..sql.selectable import ForUpdateArg

__all__ = ["Query", "QueryContext", "aliased"]


_path_registry = PathRegistry.root


@inspection._self_inspects
@log.class_logger
class Query(object):
    """ORM-level SQL construction object.

    :class:`_query.Query`
    is the source of all SELECT statements generated by the
    ORM, both those formulated by end-user query operations as well as by
    high level internal operations such as related collection loading.  It
    features a generative interface whereby successive calls return a new
    :class:`_query.Query` object, a copy of the former with additional
    criteria and options associated with it.

    :class:`_query.Query` objects are normally initially generated using the
    :meth:`~.Session.query` method of :class:`.Session`, and in
    less common cases by instantiating the :class:`_query.Query` directly and
    associating with a :class:`.Session` using the
    :meth:`_query.Query.with_session`
    method.

    For a full walkthrough of :class:`_query.Query` usage, see the
    :ref:`ormtutorial_toplevel`.

    """

    _only_return_tuples = False
    _enable_eagerloads = True
    _enable_assertions = True
    _with_labels = False
    _criterion = None
    _yield_per = None
    _order_by = False
    _group_by = False
    _having = None
    _distinct = False
    _prefixes = None
    _suffixes = None
    _offset = None
    _limit = None
    _for_update_arg = None
    _statement = None
    _correlate = frozenset()
    _populate_existing = False
    _invoke_all_eagers = True
    _version_check = False
    _autoflush = True
    _only_load_props = None
    _refresh_state = None
    _refresh_identity_token = None
    _from_obj = ()
    _join_entities = ()
    _select_from_entity = None
    _mapper_adapter_map = {}
    _filter_aliases = ()
    _from_obj_alias = None
    _joinpath = _joinpoint = util.immutabledict()
    _execution_options = util.immutabledict()
    _params = util.immutabledict()
    _attributes = util.immutabledict()
    _with_options = ()
    _with_hints = ()
    _enable_single_crit = True
    _orm_only_adapt = True
    _orm_only_from_obj_alias = True
    _current_path = _path_registry
    _has_mapper_entities = False
    _bake_ok = True

    lazy_loaded_from = None
    """An :class:`.InstanceState` that is using this :class:`_query.Query`
    for a lazy load operation.

    The primary rationale for this attribute is to support the horizontal
    sharding extension, where it is available within specific query
    execution time hooks created by this extension.   To that end, the
    attribute is only intended to be meaningful at **query execution time**,
    and importantly not any time prior to that, including query compilation
    time.

    .. note::

        Within the realm of regular :class:`_query.Query` usage,
        this attribute is
        set by the lazy loader strategy before the query is invoked.  However
        there is no established hook that is available to reliably intercept
        this value programmatically.  It is set by the lazy loading strategy
        after any  mapper option objects would have been applied, and now that
        the lazy  loading strategy in the ORM makes use of "baked" queries to
        cache SQL  compilation, the :meth:`.QueryEvents.before_compile` hook is
        also not reliable.

        Currently, setting the :paramref:`_orm.relationship.bake_queries` to
        ``False`` on the target :func:`_orm.relationship`,
        and then making use of
        the :meth:`.QueryEvents.before_compile` event hook, is the only
        available programmatic path to intercepting this attribute. In future
        releases, there will be new hooks available that allow interception of
        the :class:`_query.Query` before it is executed,
        rather than before it is
        compiled.

    .. versionadded:: 1.2.9

    """

    def __init__(self, entities, session=None):
        """Construct a :class:`_query.Query` directly.

        E.g.::

            q = Query([User, Address], session=some_session)

        The above is equivalent to::

            q = some_session.query(User, Address)

        :param entities: a sequence of entities and/or SQL expressions.

        :param session: a :class:`.Session` with which the
         :class:`_query.Query`
         will be associated.   Optional; a :class:`_query.Query`
         can be associated
         with a :class:`.Session` generatively via the
         :meth:`_query.Query.with_session` method as well.

        .. seealso::

            :meth:`.Session.query`

            :meth:`_query.Query.with_session`

        """

        self.session = session
        self._polymorphic_adapters = {}
        self._set_entities(entities)

    def _set_entities(self, entities, entity_wrapper=None):
        if entity_wrapper is None:
            entity_wrapper = _QueryEntity
        self._entities = []
        self._primary_entity = None
        self._has_mapper_entities = False

        # 1. don't run util.to_list() or _set_entity_selectables
        #    if no entities were passed - major performance bottleneck
        #    from lazy loader implementation when it seeks to use Query
        #    class for an identity lookup, causes test_orm.py to fail
        #    with thousands of extra function calls, see issue #4228
        #    for why this use had to be added
        # 2. can't use classmethod on Query because session.query_cls
        #    is an arbitrary callable in some user recipes, not
        #    necessarily a class, so we don't have the class available.
        #    see issue #4256
        # 3. can't do "if entities is not None" because we usually get here
        #    from session.query() which takes in *entities.
        # 4. can't do "if entities" because users make use of undocumented
        #    to_list() behavior here and they pass clause expressions that
        #    can't be evaluated as boolean.  See issue #4269.
        if entities != ():
            for ent in util.to_list(entities):
                entity_wrapper(self, ent)

            self._set_entity_selectables(self._entities)

    def _set_entity_selectables(self, entities):
        self._mapper_adapter_map = d = self._mapper_adapter_map.copy()

        for ent in entities:
            for entity in ent.entities:
                if entity not in d:
                    ext_info = inspect(entity)
                    if (
                        not ext_info.is_aliased_class
                        and ext_info.mapper.with_polymorphic
                    ):
                        if (
                            ext_info.mapper.persist_selectable
                            not in self._polymorphic_adapters
                        ):
                            self._mapper_loads_polymorphically_with(
                                ext_info.mapper,
                                sql_util.ColumnAdapter(
                                    ext_info.selectable,
                                    ext_info.mapper._equivalent_columns,
                                ),
                            )
                        aliased_adapter = None
                    elif ext_info.is_aliased_class:
                        aliased_adapter = ext_info._adapter
                    else:
                        aliased_adapter = None

                    d[entity] = (ext_info, aliased_adapter)
                ent.setup_entity(*d[entity])

    def _mapper_loads_polymorphically_with(self, mapper, adapter):
        for m2 in mapper._with_polymorphic_mappers or [mapper]:
            self._polymorphic_adapters[m2] = adapter
            for m in m2.iterate_to_root():
                self._polymorphic_adapters[m.local_table] = adapter

    def _set_select_from(self, obj, set_base_alias):
        fa = []
        select_from_alias = None

        for from_obj in obj:
            info = inspect(from_obj)
            if hasattr(info, "mapper") and (
                info.is_mapper or info.is_aliased_class
            ):
                self._select_from_entity = info
                if set_base_alias and not info.is_aliased_class:
                    raise sa_exc.ArgumentError(
                        "A selectable (FromClause) instance is "
                        "expected when the base alias is being set."
                    )
                fa.append(info.selectable)
            elif not info.is_selectable:
                raise sa_exc.ArgumentError(
                    "argument is not a mapped class, mapper, "
                    "aliased(), or FromClause instance."
                )
            else:
                if isinstance(from_obj, expression.SelectBase):
                    from_obj = from_obj.alias()
                if set_base_alias:
                    select_from_alias = from_obj
                fa.append(from_obj)

        self._from_obj = tuple(fa)

        if (
            set_base_alias
            and len(self._from_obj) == 1
            and isinstance(select_from_alias, expression.Alias)
        ):
            equivs = self.__all_equivs()
            self._from_obj_alias = sql_util.ColumnAdapter(
                self._from_obj[0], equivs
            )
        elif (
            set_base_alias
            and len(self._from_obj) == 1
            and hasattr(info, "mapper")
            and info.is_aliased_class
        ):
            self._from_obj_alias = info._adapter

    def _reset_polymorphic_adapter(self, mapper):
        for m2 in mapper._with_polymorphic_mappers:
            self._polymorphic_adapters.pop(m2, None)
            for m in m2.iterate_to_root():
                self._polymorphic_adapters.pop(m.local_table, None)

    def _adapt_polymorphic_element(self, element):
        if "parententity" in element._annotations:
            search = element._annotations["parententity"]
            alias = self._polymorphic_adapters.get(search, None)
            if alias:
                return alias.adapt_clause(element)

        if isinstance(element, expression.FromClause):
            search = element
        elif hasattr(element, "table"):
            search = element.table
        else:
            return None

        alias = self._polymorphic_adapters.get(search, None)
        if alias:
            return alias.adapt_clause(element)

    def _adapt_col_list(self, cols):
        return [
            self._adapt_clause(
                expression._literal_as_label_reference(o), True, True
            )
            for o in cols
        ]

    @_generative()
    def _set_lazyload_from(self, state):
        self.lazy_loaded_from = state

    @_generative()
    def _adapt_all_clauses(self):
        self._orm_only_adapt = False

    def _adapt_clause(self, clause, as_filter, orm_only):
        """Adapt incoming clauses to transformations which
        have been applied within this query."""

        adapters = []
        # do we adapt all expression elements or only those
        # tagged as 'ORM' constructs ?
        if not self._orm_only_adapt:
            orm_only = False

        if as_filter and self._filter_aliases:
            for fa in self._filter_aliases:
                adapters.append((orm_only, fa.replace))

        if self._from_obj_alias:
            # for the "from obj" alias, apply extra rule to the
            # 'ORM only' check, if this query were generated from a
            # subquery of itself, i.e. _from_selectable(), apply adaption
            # to all SQL constructs.
            adapters.append(
                (
                    orm_only if self._orm_only_from_obj_alias else False,
                    self._from_obj_alias.replace,
                )
            )

        if self._polymorphic_adapters:
            adapters.append((orm_only, self._adapt_polymorphic_element))

        if not adapters:
            return clause

        def replace(elem):
            is_orm_adapt = (
                "_orm_adapt" in elem._annotations
                or "parententity" in elem._annotations
            )
            for _orm_only, adapter in adapters:
                if not _orm_only or is_orm_adapt:
                    e = adapter(elem)
                    if e is not None:
                        return e

        return visitors.replacement_traverse(clause, {}, replace)

    def _query_entity_zero(self):
        """Return the first QueryEntity."""
        return self._entities[0]

    def _mapper_zero(self):
        """Return the Mapper associated with the first QueryEntity."""
        return self._entities[0].mapper

    def _entity_zero(self):
        """Return the 'entity' (mapper or AliasedClass) associated
        with the first QueryEntity, or alternatively the 'select from'
        entity if specified."""

        return (
            self._select_from_entity
            if self._select_from_entity is not None
            else self._query_entity_zero().entity_zero
        )

    @property
    def _mapper_entities(self):
        for ent in self._entities:
            if isinstance(ent, _MapperEntity):
                yield ent

    def _joinpoint_zero(self):
        return self._joinpoint.get("_joinpoint_entity", self._entity_zero())

    def _bind_mapper(self):
        ezero = self._entity_zero()
        if ezero is not None:
            insp = inspect(ezero)
            if not insp.is_clause_element:
                return insp.mapper

        return None

    def _only_full_mapper_zero(self, methname):
        if self._entities != [self._primary_entity]:
            raise sa_exc.InvalidRequestError(
                "%s() can only be used against "
                "a single mapped class." % methname
            )
        return self._primary_entity.entity_zero

    def _only_entity_zero(self, rationale=None):
        if len(self._entities) > 1:
            raise sa_exc.InvalidRequestError(
                rationale
                or "This operation requires a Query "
                "against a single mapper."
            )
        return self._entity_zero()

    def __all_equivs(self):
        equivs = {}
        for ent in self._mapper_entities:
            equivs.update(ent.mapper._equivalent_columns)
        return equivs

    def _get_condition(self):
        return self._no_criterion_condition(
            "get", order_by=False, distinct=False
        )

    def _get_existing_condition(self):
        self._no_criterion_assertion("get", order_by=False, distinct=False)

    def _no_criterion_assertion(self, meth, order_by=True, distinct=True):
        if not self._enable_assertions:
            return
        if (
            self._criterion is not None
            or self._statement is not None
            or self._from_obj
            or self._limit is not None
            or self._offset is not None
            or self._group_by
            or (order_by and self._order_by)
            or (distinct and self._distinct)
        ):
            raise sa_exc.InvalidRequestError(
                "Query.%s() being called on a "
                "Query with existing criterion. " % meth
            )

    def _no_criterion_condition(self, meth, order_by=True, distinct=True):
        self._no_criterion_assertion(meth, order_by, distinct)

        self._from_obj = ()
        self._statement = self._criterion = None
        self._order_by = self._group_by = self._distinct = False

    def _no_clauseelement_condition(self, meth):
        if not self._enable_assertions:
            return
        if self._order_by:
            raise sa_exc.InvalidRequestError(
                "Query.%s() being called on a "
                "Query with existing criterion. " % meth
            )
        self._no_criterion_condition(meth)

    def _no_statement_condition(self, meth):
        if not self._enable_assertions:
            return
        if self._statement is not None:
            raise sa_exc.InvalidRequestError(
                (
                    "Query.%s() being called on a Query with an existing full "
                    "statement - can't apply criterion."
                )
                % meth
            )

    def _no_limit_offset(self, meth):
        if not self._enable_assertions:
            return
        if self._limit is not None or self._offset is not None:
            raise sa_exc.InvalidRequestError(
                "Query.%s() being called on a Query which already has LIMIT "
                "or OFFSET applied. To modify the row-limited results of a "
                " Query, call from_self() first.  "
                "Otherwise, call %s() before limit() or offset() "
                "are applied." % (meth, meth)
            )

    def _get_options(
        self,
        populate_existing=None,
        version_check=None,
        only_load_props=None,
        refresh_state=None,
        identity_token=None,
    ):
        if populate_existing:
            self._populate_existing = populate_existing
        if version_check:
            self._version_check = version_check
        if refresh_state:
            self._refresh_state = refresh_state
        if only_load_props:
            self._only_load_props = set(only_load_props)
        if identity_token:
            self._refresh_identity_token = identity_token
        return self

    def _clone(self):
        cls = self.__class__
        q = cls.__new__(cls)
        q.__dict__ = self.__dict__.copy()
        return q

    @property
    def statement(self):
        """The full SELECT statement represented by this Query.

        The statement by default will not have disambiguating labels
        applied to the construct unless with_labels(True) is called
        first.

        """

        stmt = self._compile_context(labels=self._with_labels).statement
        if self._params:
            stmt = stmt.params(self._params)

        return stmt

    def subquery(self, name=None, with_labels=False, reduce_columns=False):
        """Return the full SELECT statement represented by
        this :class:`_query.Query`, embedded within an
        :class:`_expression.Alias`.

        Eager JOIN generation within the query is disabled.

        :param name: string name to be assigned as the alias;
            this is passed through to :meth:`_expression.FromClause.alias`.
            If ``None``, a name will be deterministically generated
            at compile time.

        :param with_labels: if True, :meth:`.with_labels` will be called
         on the :class:`_query.Query` first to apply table-qualified labels
         to all columns.

        :param reduce_columns: if True,
         :meth:`_expression.Select.reduce_columns` will
         be called on the resulting :func:`_expression.select` construct,
         to remove same-named columns where one also refers to the other
         via foreign key or WHERE clause equivalence.

        """
        q = self.enable_eagerloads(False)
        if with_labels:
            q = q.with_labels()
        q = q.statement

        if reduce_columns:
            q = q.reduce_columns()
        return q.alias(name=name)

    def cte(self, name=None, recursive=False):
        r"""Return the full SELECT statement represented by this
        :class:`_query.Query` represented as a common table expression (CTE).

        Parameters and usage are the same as those of the
        :meth:`_expression.SelectBase.cte` method; see that method for
        further details.

        Here is the `PostgreSQL WITH
        RECURSIVE example
        <http://www.postgresql.org/docs/8.4/static/queries-with.html>`_.
        Note that, in this example, the ``included_parts`` cte and the
        ``incl_alias`` alias of it are Core selectables, which
        means the columns are accessed via the ``.c.`` attribute.  The
        ``parts_alias`` object is an :func:`_orm.aliased` instance of the
        ``Part`` entity, so column-mapped attributes are available
        directly::

            from sqlalchemy.orm import aliased

            class Part(Base):
                __tablename__ = 'part'
                part = Column(String, primary_key=True)
                sub_part = Column(String, primary_key=True)
                quantity = Column(Integer)

            included_parts = session.query(
                            Part.sub_part,
                            Part.part,
                            Part.quantity).\
                                filter(Part.part=="our part").\
                                cte(name="included_parts", recursive=True)

            incl_alias = aliased(included_parts, name="pr")
            parts_alias = aliased(Part, name="p")
            included_parts = included_parts.union_all(
                session.query(
                    parts_alias.sub_part,
                    parts_alias.part,
                    parts_alias.quantity).\
                        filter(parts_alias.part==incl_alias.c.sub_part)
                )

            q = session.query(
                    included_parts.c.sub_part,
                    func.sum(included_parts.c.quantity).
                        label('total_quantity')
                ).\
                group_by(included_parts.c.sub_part)

        .. seealso::

            :meth:`_expression.HasCTE.cte`

        """
        return self.enable_eagerloads(False).statement.cte(
            name=name, recursive=recursive
        )

    def label(self, name):
        """Return the full SELECT statement represented by this
        :class:`_query.Query`, converted
        to a scalar subquery with a label of the given name.

        Analogous to :meth:`_expression.SelectBase.label`.

        """

        return self.enable_eagerloads(False).statement.label(name)

    def as_scalar(self):
        """Return the full SELECT statement represented by this
        :class:`_query.Query`, converted to a scalar subquery.

        Analogous to :meth:`_expression.SelectBase.as_scalar`.

        """

        return self.enable_eagerloads(False).statement.as_scalar()

    @property
    def selectable(self):
        """Return the :class:`_expression.Select` object emitted by this
        :class:`_query.Query`.

        Used for :func:`_sa.inspect` compatibility, this is equivalent to::

            query.enable_eagerloads(False).with_labels().statement

        """
        return self.__clause_element__()

    def __clause_element__(self):
        return self.enable_eagerloads(False).with_labels().statement

    @_generative()
    def only_return_tuples(self, value):
        """When set to True, the query results will always be a tuple.

        This is specifically for single element queries. The default is False.

        .. versionadded:: 1.2.5

        .. seealso::

            :meth:`_query.Query.is_single_entity`

        """
        self._only_return_tuples = value

    @property
    def is_single_entity(self):
        """Indicates if this :class:`_query.Query`
        returns tuples or single entities.

        Returns True if this query returns a single entity for each instance
        in its result list, and False if this query returns a tuple of entities
        for each result.

        .. versionadded:: 1.3.11

        .. seealso::

            :meth:`_query.Query.only_return_tuples`

        """
        return (
            not self._only_return_tuples
            and len(self._entities) == 1
            and self._entities[0].supports_single_entity
        )

    @_generative()
    def enable_eagerloads(self, value):
        """Control whether or not eager joins and subqueries are
        rendered.

        When set to False, the returned Query will not render
        eager joins regardless of :func:`~sqlalchemy.orm.joinedload`,
        :func:`~sqlalchemy.orm.subqueryload` options
        or mapper-level ``lazy='joined'``/``lazy='subquery'``
        configurations.

        This is used primarily when nesting the Query's
        statement into a subquery or other
        selectable, or when using :meth:`_query.Query.yield_per`.

        """
        self._enable_eagerloads = value

    def _no_yield_per(self, message):
        raise sa_exc.InvalidRequestError(
            "The yield_per Query option is currently not "
            "compatible with %s eager loading.  Please "
            "specify lazyload('*') or query.enable_eagerloads(False) in "
            "order to "
            "proceed with query.yield_per()." % message
        )

    @_generative()
    def with_labels(self):
        """Apply column labels to the return value of Query.statement.

        Indicates that this Query's `statement` accessor should return
        a SELECT statement that applies labels to all columns in the
        form <tablename>_<columnname>; this is commonly used to
        disambiguate columns from multiple tables which have the same
        name.

        When the `Query` actually issues SQL to load rows, it always
        uses column labeling.

        .. note:: The :meth:`_query.Query.with_labels` method *only* applies
           the output of :attr:`_query.Query.statement`, and *not* to any of
           the result-row invoking systems of :class:`_query.Query` itself,
           e.g. :meth:`_query.Query.first`, :meth:`_query.Query.all`, etc.
           To execute
           a query using :meth:`_query.Query.with_labels`, invoke the
           :attr:`_query.Query.statement` using :meth:`.Session.execute`::

                result = session.execute(query.with_labels().statement)


        """
        self._with_labels = True

    @_generative()
    def enable_assertions(self, value):
        """Control whether assertions are generated.

        When set to False, the returned Query will
        not assert its state before certain operations,
        including that LIMIT/OFFSET has not been applied
        when filter() is called, no criterion exists
        when get() is called, and no "from_statement()"
        exists when filter()/order_by()/group_by() etc.
        is called.  This more permissive mode is used by
        custom Query subclasses to specify criterion or
        other modifiers outside of the usual usage patterns.

        Care should be taken to ensure that the usage
        pattern is even possible.  A statement applied
        by from_statement() will override any criterion
        set by filter() or order_by(), for example.

        """
        self._enable_assertions = value

    @property
    def whereclause(self):
        """A readonly attribute which returns the current WHERE criterion for
        this Query.

        This returned value is a SQL expression construct, or ``None`` if no
        criterion has been established.

        """
        return self._criterion

    @_generative()
    def _with_current_path(self, path):
        """Indicate that this query applies to objects loaded
        within a certain path.

        Used by deferred loaders (see strategies.py) which transfer
        query options from an originating query to a newly generated
        query intended for the deferred load.

        """
        self._current_path = path

    @_generative(_no_clauseelement_condition)
    def with_polymorphic(
        self, cls_or_mappers, selectable=None, polymorphic_on=None
    ):
        """Load columns for inheriting classes.

        :meth:`_query.Query.with_polymorphic` applies transformations
        to the "main" mapped class represented by this :class:`_query.Query`.
        The "main" mapped class here means the :class:`_query.Query`
        object's first argument is a full class, i.e.
        ``session.query(SomeClass)``. These transformations allow additional
        tables to be present in the FROM clause so that columns for a
        joined-inheritance subclass are available in the query, both for the
        purposes of load-time efficiency as well as the ability to use
        these columns at query time.

        See the documentation section :ref:`with_polymorphic` for
        details on how this method is used.

        """

        if not self._primary_entity:
            raise sa_exc.InvalidRequestError(
                "No primary mapper set up for this Query."
            )
        entity = self._entities[0]._clone()
        self._entities = [entity] + self._entities[1:]
        entity.set_with_polymorphic(
            self,
            cls_or_mappers,
            selectable=selectable,
            polymorphic_on=polymorphic_on,
        )

    @_generative()
    def yield_per(self, count):
        r"""Yield only ``count`` rows at a time.

        The purpose of this method is when fetching very large result sets
        (> 10K rows), to batch results in sub-collections and yield them
        out partially, so that the Python interpreter doesn't need to declare
        very large areas of memory which is both time consuming and leads
        to excessive memory use.   The performance from fetching hundreds of
        thousands of rows can often double when a suitable yield-per setting
        (e.g. approximately 1000) is used, even with DBAPIs that buffer
        rows (which are most).

        The :meth:`_query.Query.yield_per` method **is not compatible
        subqueryload eager loading or joinedload eager loading when
        using collections**.  It is potentially compatible with "select in"
        eager loading, **provided the database driver supports multiple,
        independent cursors** (pysqlite and psycopg2 are known to work,
        MySQL and SQL Server ODBC drivers do not).

        Therefore in some cases, it may be helpful to disable
        eager loads, either unconditionally with
        :meth:`_query.Query.enable_eagerloads`::

            q = sess.query(Object).yield_per(100).enable_eagerloads(False)

        Or more selectively using :func:`.lazyload`; such as with
        an asterisk to specify the default loader scheme::

            q = sess.query(Object).yield_per(100).\
                options(lazyload('*'), joinedload(Object.some_related))

        .. warning::

            Use this method with caution; if the same instance is
            present in more than one batch of rows, end-user changes
            to attributes will be overwritten.

            In particular, it's usually impossible to use this setting
            with eagerly loaded collections (i.e. any lazy='joined' or
            'subquery') since those collections will be cleared for a
            new load when encountered in a subsequent result batch.
            In the case of 'subquery' loading, the full result for all
            rows is fetched which generally defeats the purpose of
            :meth:`~sqlalchemy.orm.query.Query.yield_per`.

            Also note that while
            :meth:`~sqlalchemy.orm.query.Query.yield_per` will set the
            ``stream_results`` execution option to True, currently
            this is only understood by
            :mod:`~sqlalchemy.dialects.postgresql.psycopg2`,
            :mod:`~sqlalchemy.dialects.mysql.mysqldb` and
            :mod:`~sqlalchemy.dialects.mysql.pymysql` dialects
            which will stream results using server side cursors
            instead of pre-buffer all rows for this query. Other
            DBAPIs **pre-buffer all rows** before making them
            available.  The memory use of raw database rows is much less
            than that of an ORM-mapped object, but should still be taken into
            consideration when benchmarking.

        .. seealso::

            :meth:`_query.Query.enable_eagerloads`

        """
        self._yield_per = count
        self._execution_options = self._execution_options.union(
            {"stream_results": True, "max_row_buffer": count}
        )

    def get(self, ident):
        """Return an instance based on the given primary key identifier,
        or ``None`` if not found.

        E.g.::

            my_user = session.query(User).get(5)

            some_object = session.query(VersionedFoo).get((5, 10))

            some_object = session.query(VersionedFoo).get(
                {"id": 5, "version_id": 10})

        :meth:`_query.Query.get` is special in that it provides direct
        access to the identity map of the owning :class:`.Session`.
        If the given primary key identifier is present
        in the local identity map, the object is returned
        directly from this collection and no SQL is emitted,
        unless the object has been marked fully expired.
        If not present,
        a SELECT is performed in order to locate the object.

        :meth:`_query.Query.get` also will perform a check if
        the object is present in the identity map and
        marked as expired - a SELECT
        is emitted to refresh the object as well as to
        ensure that the row is still present.
        If not, :class:`~sqlalchemy.orm.exc.ObjectDeletedError` is raised.

        :meth:`_query.Query.get` is only used to return a single
        mapped instance, not multiple instances or
        individual column constructs, and strictly
        on a single primary key value.  The originating
        :class:`_query.Query` must be constructed in this way,
        i.e. against a single mapped entity,
        with no additional filtering criterion.  Loading
        options via :meth:`_query.Query.options` may be applied
        however, and will be used if the object is not
        yet locally present.

        A lazy-loading, many-to-one attribute configured
        by :func:`_orm.relationship`, using a simple
        foreign-key-to-primary-key criterion, will also use an
        operation equivalent to :meth:`_query.Query.get` in order to retrieve
        the target value from the local identity map
        before querying the database.  See :doc:`/orm/loading_relationships`
        for further details on relationship loading.

        :param ident: A scalar, tuple, or dictionary representing the
         primary key.  For a composite (e.g. multiple column) primary key,
         a tuple or dictionary should be passed.

         For a single-column primary key, the scalar calling form is typically
         the most expedient.  If the primary key of a row is the value "5",
         the call looks like::

            my_object = query.get(5)

         The tuple form contains primary key values typically in
         the order in which they correspond to the mapped
         :class:`_schema.Table`
         object's primary key columns, or if the
         :paramref:`_orm.Mapper.primary_key` configuration parameter were
         used, in
         the order used for that parameter. For example, if the primary key
         of a row is represented by the integer
         digits "5, 10" the call would look like::

             my_object = query.get((5, 10))

         The dictionary form should include as keys the mapped attribute names
         corresponding to each element of the primary key.  If the mapped class
         has the attributes ``id``, ``version_id`` as the attributes which
         store the object's primary key value, the call would look like::

            my_object = query.get({"id": 5, "version_id": 10})

         .. versionadded:: 1.3 the :meth:`_query.Query.get`
            method now optionally
            accepts a dictionary of attribute names to values in order to
            indicate a primary key identifier.


        :return: The object instance, or ``None``.

        """
        return self._get_impl(ident, loading.load_on_pk_identity)

    def _identity_lookup(
        self,
        mapper,
        primary_key_identity,
        identity_token=None,
        passive=attributes.PASSIVE_OFF,
        lazy_loaded_from=None,
    ):
        """Locate an object in the identity map.

        Given a primary key identity, constructs an identity key and then
        looks in the session's identity map.  If present, the object may
        be run through unexpiration rules (e.g. load unloaded attributes,
        check if was deleted).

        For performance reasons, while the :class:`_query.Query` must be
        instantiated, it may be instantiated with no entities, and the
        mapper is passed::

            obj = session.query()._identity_lookup(inspect(SomeClass), (1, ))

        :param mapper: mapper in use
        :param primary_key_identity: the primary key we are searching for, as
         a tuple.
        :param identity_token: identity token that should be used to create
         the identity key.  Used as is, however overriding subclasses can
         repurpose this in order to interpret the value in a special way,
         such as if None then look among multiple target tokens.
        :param passive: passive load flag passed to
         :func:`.loading.get_from_identity`, which impacts the behavior if
         the object is found; the object may be validated and/or unexpired
         if the flag allows for SQL to be emitted.
        :param lazy_loaded_from: an :class:`.InstanceState` that is
         specifically asking for this identity as a related identity.  Used
         for sharding schemes where there is a correspondence between an object
         and a related object being lazy-loaded (or otherwise
         relationship-loaded).

         .. versionadded:: 1.2.9

        :return: None if the object is not found in the identity map, *or*
         if the object was unexpired and found to have been deleted.
         if passive flags disallow SQL and the object is expired, returns
         PASSIVE_NO_RESULT.   In all other cases the instance is returned.

        .. versionadded:: 1.2.7

        """

        key = mapper.identity_key_from_primary_key(
            primary_key_identity, identity_token=identity_token
        )
        return loading.get_from_identity(self.session, mapper, key, passive)

    def _get_impl(self, primary_key_identity, db_load_fn, identity_token=None):
        # convert composite types to individual args
        if hasattr(primary_key_identity, "__composite_values__"):
            primary_key_identity = primary_key_identity.__composite_values__()

        mapper = self._only_full_mapper_zero("get")

        is_dict = isinstance(primary_key_identity, dict)
        if not is_dict:
            primary_key_identity = util.to_list(
                primary_key_identity, default=(None,)
            )

        if len(primary_key_identity) != len(mapper.primary_key):
            raise sa_exc.InvalidRequestError(
                "Incorrect number of values in identifier to formulate "
                "primary key for query.get(); primary key columns are %s"
                % ",".join("'%s'" % c for c in mapper.primary_key)
            )

        if is_dict:
            try:
                primary_key_identity = list(
                    primary_key_identity[prop.key]
                    for prop in mapper._identity_key_props
                )

            except KeyError as err:
                util.raise_(
                    sa_exc.InvalidRequestError(
                        "Incorrect names of values in identifier to formulate "
                        "primary key for query.get(); primary key attribute "
                        "names are %s"
                        % ",".join(
                            "'%s'" % prop.key
                            for prop in mapper._identity_key_props
                        )
                    ),
                    replace_context=err,
                )

        if (
            not self._populate_existing
            and not mapper.always_refresh
            and self._for_update_arg is None
        ):

            instance = self._identity_lookup(
                mapper, primary_key_identity, identity_token=identity_token
            )

            if instance is not None:
                self._get_existing_condition()
                # reject calls for id in identity map but class
                # mismatch.
                if not issubclass(instance.__class__, mapper.class_):
                    return None
                return instance
            elif instance is attributes.PASSIVE_CLASS_MISMATCH:
                return None

        return db_load_fn(self, primary_key_identity)

    @_generative()
    def correlate(self, *args):
        """Return a :class:`.Query` construct which will correlate the given
        FROM clauses to that of an enclosing :class:`.Query` or
        :func:`~.expression.select`.

        The method here accepts mapped classes, :func:`.aliased` constructs,
        and :func:`.mapper` constructs as arguments, which are resolved into
        expression constructs, in addition to appropriate expression
        constructs.

        The correlation arguments are ultimately passed to
        :meth:`_expression.Select.correlate`
        after coercion to expression constructs.

        The correlation arguments take effect in such cases
        as when :meth:`_query.Query.from_self` is used, or when
        a subquery as returned by :meth:`_query.Query.subquery` is
        embedded in another :func:`_expression.select` construct.

        """

        for s in args:
            if s is None:
                self._correlate = self._correlate.union([None])
            else:
                self._correlate = self._correlate.union(
                    sql_util.surface_selectables(_interpret_as_from(s))
                )

    @_generative()
    def autoflush(self, setting):
        """Return a Query with a specific 'autoflush' setting.

        Note that a Session with autoflush=False will
        not autoflush, even if this flag is set to True at the
        Query level.  Therefore this flag is usually used only
        to disable autoflush for a specific Query.

        """
        self._autoflush = setting

    @_generative()
    def populate_existing(self):
        """Return a :class:`_query.Query`
        that will expire and refresh all instances
        as they are loaded, or reused from the current :class:`.Session`.

        :meth:`.populate_existing` does not improve behavior when
        the ORM is used normally - the :class:`.Session` object's usual
        behavior of maintaining a transaction and expiring all attributes
        after rollback or commit handles object state automatically.
        This method is not intended for general use.

        .. seealso::

            :ref:`session_expire` - in the ORM :class:`_orm.Session`
            documentation

        """
        self._populate_existing = True

    @_generative()
    def _with_invoke_all_eagers(self, value):
        """Set the 'invoke all eagers' flag which causes joined- and
        subquery loaders to traverse into already-loaded related objects
        and collections.

        Default is that of :attr:`_query.Query._invoke_all_eagers`.

        """
        self._invoke_all_eagers = value

    def with_parent(self, instance, property=None, from_entity=None):  # noqa
        """Add filtering criterion that relates the given instance
        to a child object or collection, using its attribute state
        as well as an established :func:`_orm.relationship()`
        configuration.

        The method uses the :func:`.with_parent` function to generate
        the clause, the result of which is passed to
        :meth:`_query.Query.filter`.

        Parameters are the same as :func:`.with_parent`, with the exception
        that the given property can be None, in which case a search is
        performed against this :class:`_query.Query` object's target mapper.

        :param instance:
          An instance which has some :func:`_orm.relationship`.

        :param property:
          String property name, or class-bound attribute, which indicates
          what relationship from the instance should be used to reconcile the
          parent/child relationship.

        :param from_entity:
          Entity in which to consider as the left side.  This defaults to the
          "zero" entity of the :class:`_query.Query` itself.

        """

        if from_entity:
            entity_zero = inspect(from_entity)
        else:
            entity_zero = self._entity_zero()
        if property is None:

            mapper = object_mapper(instance)

            for prop in mapper.iterate_properties:
                if (
                    isinstance(prop, properties.RelationshipProperty)
                    and prop.mapper is entity_zero.mapper
                ):
                    property = prop  # noqa
                    break
            else:
                raise sa_exc.InvalidRequestError(
                    "Could not locate a property which relates instances "
                    "of class '%s' to instances of class '%s'"
                    % (
                        entity_zero.mapper.class_.__name__,
                        instance.__class__.__name__,
                    )
                )

        return self.filter(with_parent(instance, property, entity_zero.entity))

    @_generative()
    def add_entity(self, entity, alias=None):
        """add a mapped entity to the list of result columns
        to be returned."""

        if alias is not None:
            entity = aliased(entity, alias)

        self._entities = list(self._entities)
        m = _MapperEntity(self, entity)
        self._set_entity_selectables([m])

    @_generative()
    def with_session(self, session):
        """Return a :class:`_query.Query` that will use the given
        :class:`.Session`.

        While the :class:`_query.Query`
        object is normally instantiated using the
        :meth:`.Session.query` method, it is legal to build the
        :class:`_query.Query`
        directly without necessarily using a :class:`.Session`.  Such a
        :class:`_query.Query` object, or any :class:`_query.Query`
        already associated
        with a different :class:`.Session`, can produce a new
        :class:`_query.Query`
        object associated with a target session using this method::

            from sqlalchemy.orm import Query

            query = Query([MyClass]).filter(MyClass.id == 5)

            result = query.with_session(my_session).one()

        """

        self.session = session

    def from_self(self, *entities):
        r"""return a Query that selects from this Query's
        SELECT statement.

        :meth:`_query.Query.from_self` essentially turns the SELECT statement
        into a SELECT of itself.  Given a query such as::

            q = session.query(User).filter(User.name.like('e%'))

        Given the :meth:`_query.Query.from_self` version::

            q = session.query(User).filter(User.name.like('e%')).from_self()

        This query renders as:

        .. sourcecode:: sql

            SELECT anon_1.user_id AS anon_1_user_id,
                   anon_1.user_name AS anon_1_user_name
            FROM (SELECT "user".id AS user_id, "user".name AS user_name
            FROM "user"
            WHERE "user".name LIKE :name_1) AS anon_1

        There are lots of cases where :meth:`_query.Query.from_self`
        may be useful.
        A simple one is where above, we may want to apply a row LIMIT to
        the set of user objects we query against, and then apply additional
        joins against that row-limited set::

            q = session.query(User).filter(User.name.like('e%')).\
                limit(5).from_self().\
                join(User.addresses).filter(Address.email.like('q%'))

        The above query joins to the ``Address`` entity but only against the
        first five results of the ``User`` query:

        .. sourcecode:: sql

            SELECT anon_1.user_id AS anon_1_user_id,
                   anon_1.user_name AS anon_1_user_name
            FROM (SELECT "user".id AS user_id, "user".name AS user_name
            FROM "user"
            WHERE "user".name LIKE :name_1
             LIMIT :param_1) AS anon_1
            JOIN address ON anon_1.user_id = address.user_id
            WHERE address.email LIKE :email_1

        **Automatic Aliasing**

        Another key behavior of :meth:`_query.Query.from_self`
        is that it applies
        **automatic aliasing** to the entities inside the subquery, when
        they are referenced on the outside.  Above, if we continue to
        refer to the ``User`` entity without any additional aliasing applied
        to it, those references wil be in terms of the subquery::

            q = session.query(User).filter(User.name.like('e%')).\
                limit(5).from_self().\
                join(User.addresses).filter(Address.email.like('q%')).\
                order_by(User.name)

        The ORDER BY against ``User.name`` is aliased to be in terms of the
        inner subquery:

        .. sourcecode:: sql

            SELECT anon_1.user_id AS anon_1_user_id,
                   anon_1.user_name AS anon_1_user_name
            FROM (SELECT "user".id AS user_id, "user".name AS user_name
            FROM "user"
            WHERE "user".name LIKE :name_1
             LIMIT :param_1) AS anon_1
            JOIN address ON anon_1.user_id = address.user_id
            WHERE address.email LIKE :email_1 ORDER BY anon_1.user_name

        The automatic aliasing feature only works in a **limited** way,
        for simple filters and orderings.   More ambitious constructions
        such as referring to the entity in joins should prefer to use
        explicit subquery objects, typically making use of the
        :meth:`_query.Query.subquery`
        method to produce an explicit subquery object.
        Always test the structure of queries by viewing the SQL to ensure
        a particular structure does what's expected!

        **Changing the Entities**

        :meth:`_query.Query.from_self`
        also includes the ability to modify what
        columns are being queried.   In our example, we want ``User.id``
        to be queried by the inner query, so that we can join to the
        ``Address`` entity on the outside, but we only wanted the outer
        query to return the ``Address.email`` column::

            q = session.query(User).filter(User.name.like('e%')).\
                limit(5).from_self(Address.email).\
                join(User.addresses).filter(Address.email.like('q%'))

        yielding:

        .. sourcecode:: sql

            SELECT address.email AS address_email
            FROM (SELECT "user".id AS user_id, "user".name AS user_name
            FROM "user"
            WHERE "user".name LIKE :name_1
             LIMIT :param_1) AS anon_1
            JOIN address ON anon_1.user_id = address.user_id
            WHERE address.email LIKE :email_1

        **Looking out for Inner / Outer Columns**

        Keep in mind that when referring to columns that originate from
        inside the subquery, we need to ensure they are present in the
        columns clause of the subquery itself; this is an ordinary aspect of
        SQL.  For example, if we wanted to load from a joined entity inside
        the subquery using :func:`.contains_eager`, we need to add those
        columns.   Below illustrates a join of ``Address`` to ``User``,
        then a subquery, and then we'd like :func:`.contains_eager` to access
        the ``User`` columns::

            q = session.query(Address).join(Address.user).\
                filter(User.name.like('e%'))

            q = q.add_entity(User).from_self().\
                options(contains_eager(Address.user))

        We use :meth:`_query.Query.add_entity` above **before** we call
        :meth:`_query.Query.from_self`
        so that the ``User`` columns are present
        in the inner subquery, so that they are available to the
        :func:`.contains_eager` modifier we are using on the outside,
        producing:

        .. sourcecode:: sql

            SELECT anon_1.address_id AS anon_1_address_id,
                   anon_1.address_email AS anon_1_address_email,
                   anon_1.address_user_id AS anon_1_address_user_id,
                   anon_1.user_id AS anon_1_user_id,
                   anon_1.user_name AS anon_1_user_name
            FROM (
                SELECT address.id AS address_id,
                address.email AS address_email,
                address.user_id AS address_user_id,
                "user".id AS user_id,
                "user".name AS user_name
            FROM address JOIN "user" ON "user".id = address.user_id
            WHERE "user".name LIKE :name_1) AS anon_1

        If we didn't call ``add_entity(User)``, but still asked
        :func:`.contains_eager` to load the ``User`` entity, it would be
        forced to add the table on the outside without the correct
        join criteria - note the ``anon1, "user"`` phrase at
        the end:

        .. sourcecode:: sql

            -- incorrect query
            SELECT anon_1.address_id AS anon_1_address_id,
                   anon_1.address_email AS anon_1_address_email,
                   anon_1.address_user_id AS anon_1_address_user_id,
                   "user".id AS user_id,
                   "user".name AS user_name
            FROM (
                SELECT address.id AS address_id,
                address.email AS address_email,
                address.user_id AS address_user_id
            FROM address JOIN "user" ON "user".id = address.user_id
            WHERE "user".name LIKE :name_1) AS anon_1, "user"

        :param \*entities: optional list of entities which will replace
         those being selected.

        """

        fromclause = (
            self.with_labels()
            .enable_eagerloads(False)
            .statement.correlate(None)
        )
        q = self._from_selectable(fromclause)
        q._enable_single_crit = False
        q._select_from_entity = self._entity_zero()
        if entities:
            q._set_entities(entities)
        return q

    @_generative()
    def _set_enable_single_crit(self, val):
        self._enable_single_crit = val

    @_generative()
    def _from_selectable(self, fromclause):
        for attr in (
            "_statement",
            "_criterion",
            "_order_by",
            "_group_by",
            "_limit",
            "_offset",
            "_joinpath",
            "_joinpoint",
            "_distinct",
            "_having",
            "_prefixes",
            "_suffixes",
        ):
            self.__dict__.pop(attr, None)
        self._set_select_from([fromclause], True)

        # this enables clause adaptation for non-ORM
        # expressions.
        self._orm_only_from_obj_alias = False

        old_entities = self._entities
        self._entities = []
        for e in old_entities:
            e.adapt_to_selectable(self, self._from_obj[0])

    def values(self, *columns):
        """Return an iterator yielding result tuples corresponding
        to the given list of columns.

        """

        if not columns:
            return iter(())
        q = self._clone()
        q._set_entities(columns, entity_wrapper=_ColumnEntity)
        if not q._yield_per:
            q._yield_per = 10
        return iter(q)

    _values = values

    def value(self, column):
        """Return a scalar result corresponding to the given
        column expression.

        """
        try:
            return next(self.values(column))[0]
        except StopIteration:
            return None

    @_generative()
    def with_entities(self, *entities):
        r"""Return a new :class:`_query.Query`
        replacing the SELECT list with the
        given entities.

        e.g.::

            # Users, filtered on some arbitrary criterion
            # and then ordered by related email address
            q = session.query(User).\
                        join(User.address).\
                        filter(User.name.like('%ed%')).\
                        order_by(Address.email)

            # given *only* User.id==5, Address.email, and 'q', what
            # would the *next* User in the result be ?
            subq = q.with_entities(Address.email).\
                        order_by(None).\
                        filter(User.id==5).\
                        subquery()
            q = q.join((subq, subq.c.email < Address.email)).\
                        limit(1)

        """
        self._set_entities(entities)

    @_generative()
    def add_columns(self, *column):
        """Add one or more column expressions to the list
        of result columns to be returned."""

        self._entities = list(self._entities)
        l = len(self._entities)
        for c in column:
            _ColumnEntity(self, c)
        # _ColumnEntity may add many entities if the
        # given arg is a FROM clause
        self._set_entity_selectables(self._entities[l:])

    @util.pending_deprecation(
        "0.7",
        ":meth:`.add_column` is superseded " "by :meth:`.add_columns`",
        False,
    )
    def add_column(self, column):
        """Add a column expression to the list of result columns to be
        returned.

        Pending deprecation: :meth:`.add_column` will be superseded by
        :meth:`.add_columns`.

        """
        return self.add_columns(column)

    def options(self, *args):
        """Return a new :class:`_query.Query` object,
        applying the given list of
        mapper options.

        Most supplied options regard changing how column- and
        relationship-mapped attributes are loaded.

        .. seealso::

            :ref:`deferred_options`

            :ref:`relationship_loader_options`

        """

        return self._options(False, *args)

    def _conditional_options(self, *args):
        return self._options(True, *args)

    @_generative()
    def _options(self, conditional, *args):
        # most MapperOptions write to the '_attributes' dictionary,
        # so copy that as well
        self._attributes = self._attributes.copy()
        if "_unbound_load_dedupes" not in self._attributes:
            self._attributes["_unbound_load_dedupes"] = set()
        opts = tuple(util.flatten_iterator(args))
        self._with_options = self._with_options + opts
        if conditional:
            for opt in opts:
                opt.process_query_conditionally(self)
        else:
            for opt in opts:
                opt.process_query(self)

    def with_transformation(self, fn):
        """Return a new :class:`_query.Query` object transformed by
        the given function.

        E.g.::

            def filter_something(criterion):
                def transform(q):
                    return q.filter(criterion)
                return transform

            q = q.with_transformation(filter_something(x==5))

        This allows ad-hoc recipes to be created for :class:`_query.Query`
        objects.  See the example at :ref:`hybrid_transformers`.

        """
        return fn(self)

    @_generative()
    def with_hint(self, selectable, text, dialect_name="*"):
        """Add an indexing or other executional context
        hint for the given entity or selectable to
        this :class:`_query.Query`.

        Functionality is passed straight through to
        :meth:`_expression.Select.with_hint`,
        with the addition that ``selectable`` can be a
        :class:`_schema.Table`, :class:`_expression.Alias`,
        or ORM entity / mapped class
        /etc.

        .. seealso::

            :meth:`_query.Query.with_statement_hint`

            :meth:`.Query.prefix_with` - generic SELECT prefixing which also
            can suit some database-specific HINT syntaxes such as MySQL
            optimizer hints

        """

        if selectable is not None:
            selectable = inspect(selectable).selectable

        self._with_hints += ((selectable, text, dialect_name),)

    def with_statement_hint(self, text, dialect_name="*"):
        """Add a statement hint to this :class:`_expression.Select`.

        This method is similar to :meth:`_expression.Select.with_hint`
        except that
        it does not require an individual table, and instead applies to the
        statement as a whole.

        This feature calls down into
        :meth:`_expression.Select.with_statement_hint`.

        .. versionadded:: 1.0.0

        .. seealso::

            :meth:`_query.Query.with_hint`

        """
        return self.with_hint(None, text, dialect_name)

    def get_execution_options(self):
        """Get the non-SQL options which will take effect during execution.

        .. versionadded:: 1.3

        .. seealso::

            :meth:`_query.Query.execution_options`
        """
        return self._execution_options

    @_generative()
    def execution_options(self, **kwargs):
        """Set non-SQL options which take effect during execution.

        The options are the same as those accepted by
        :meth:`_engine.Connection.execution_options`.

        Note that the ``stream_results`` execution option is enabled
        automatically if the :meth:`~sqlalchemy.orm.query.Query.yield_per()`
        method is used.

        .. seealso::

            :meth:`_query.Query.get_execution_options`

        """
        self._execution_options = self._execution_options.union(kwargs)

    @_generative()
    @util.deprecated(
        "0.9",
        "The :meth:`_query.Query.with_lockmode` "
        "method is deprecated and will "
        "be removed in a future release.  Please refer to "
        ":meth:`_query.Query.with_for_update`. ",
    )
    def with_lockmode(self, mode):
        """Return a new :class:`_query.Query`
        object with the specified "locking mode",
        which essentially refers to the ``FOR UPDATE`` clause.

        :param mode: a string representing the desired locking mode.
         Valid values are:

         * ``None`` - translates to no lockmode

         * ``'update'`` - translates to ``FOR UPDATE``
           (standard SQL, supported by most dialects)

         * ``'update_nowait'`` - translates to ``FOR UPDATE NOWAIT``
           (supported by Oracle, PostgreSQL 8.1 upwards)

         * ``'read'`` - translates to ``LOCK IN SHARE MODE`` (for MySQL),
           and ``FOR SHARE`` (for PostgreSQL)

        .. seealso::

            :meth:`_query.Query.with_for_update` - improved API for
            specifying the ``FOR UPDATE`` clause.

        """
        self._for_update_arg = LockmodeArg.parse_legacy_query(mode)

    @_generative()
    def with_for_update(
        self,
        read=False,
        nowait=False,
        of=None,
        skip_locked=False,
        key_share=False,
    ):
        """return a new :class:`_query.Query`
        with the specified options for the
        ``FOR UPDATE`` clause.

        The behavior of this method is identical to that of
        :meth:`_expression.GenerativeSelect.with_for_update`.
        When called with no arguments,
        the resulting ``SELECT`` statement will have a ``FOR UPDATE`` clause
        appended.  When additional arguments are specified, backend-specific
        options such as ``FOR UPDATE NOWAIT`` or ``LOCK IN SHARE MODE``
        can take effect.

        E.g.::

            q = sess.query(User).populate_existing().with_for_update(nowait=True, of=User)

        The above query on a PostgreSQL backend will render like::

            SELECT users.id AS users_id FROM users FOR UPDATE OF users NOWAIT

        .. versionadded:: 0.9.0 :meth:`_query.Query.with_for_update`
           supersedes
           the :meth:`_query.Query.with_lockmode` method.

        .. note::  It is generally a good idea to combine the use of the
           :meth:`_orm.Query.populate_existing` method when using the
           :meth:`_orm.Query.with_for_update` method.   The purpose of
           :meth:`_orm.Query.populate_existing` is to force all the data read
           from the SELECT to be populated into the ORM objects returned,
           even if these objects are already in the :term:`identity map`.

        .. seealso::

            :meth:`_expression.GenerativeSelect.with_for_update`
            - Core level method with
            full argument and behavioral description.

            :meth:`_orm.Query.populate_existing` - overwrites attributes of
            objects already loaded in the identity map.

        """  # noqa: E501

        self._for_update_arg = LockmodeArg(
            read=read,
            nowait=nowait,
            of=of,
            skip_locked=skip_locked,
            key_share=key_share,
        )

    @_generative()
    def params(self, *args, **kwargs):
        r"""Add values for bind parameters which may have been
        specified in filter().

        Parameters may be specified using \**kwargs, or optionally a single
        dictionary as the first positional argument. The reason for both is
        that \**kwargs is convenient, however some parameter dictionaries
        contain unicode keys in which case \**kwargs cannot be used.

        """
        if len(args) == 1:
            kwargs.update(args[0])
        elif len(args) > 0:
            raise sa_exc.ArgumentError(
                "params() takes zero or one positional argument, "
                "which is a dictionary."
            )
        self._params = self._params.copy()
        self._params.update(kwargs)

    @_generative(_no_statement_condition, _no_limit_offset)
    def filter(self, *criterion):
        r"""Apply the given filtering criterion to a copy
        of this :class:`_query.Query`, using SQL expressions.

        e.g.::

            session.query(MyClass).filter(MyClass.name == 'some name')

        Multiple criteria may be specified as comma separated; the effect
        is that they will be joined together using the :func:`.and_`
        function::

            session.query(MyClass).\
                filter(MyClass.name == 'some name', MyClass.id > 5)

        The criterion is any SQL expression object applicable to the
        WHERE clause of a select.   String expressions are coerced
        into SQL expression constructs via the :func:`_expression.text`
        construct.

        .. seealso::

            :meth:`_query.Query.filter_by` - filter on keyword expressions.

        """
        for criterion in list(criterion):
            criterion = expression._expression_literal_as_text(criterion)

            criterion = self._adapt_clause(criterion, True, True)

            if self._criterion is not None:
                self._criterion = self._criterion & criterion
            else:
                self._criterion = criterion

    def filter_by(self, **kwargs):
        r"""Apply the given filtering criterion to a copy
        of this :class:`_query.Query`, using keyword expressions.

        e.g.::

            session.query(MyClass).filter_by(name = 'some name')

        Multiple criteria may be specified as comma separated; the effect
        is that they will be joined together using the :func:`.and_`
        function::

            session.query(MyClass).\
                filter_by(name = 'some name', id = 5)

        The keyword expressions are extracted from the primary
        entity of the query, or the last entity that was the
        target of a call to :meth:`_query.Query.join`.

        .. seealso::

            :meth:`_query.Query.filter` - filter on SQL expressions.

        """

        zero = self._joinpoint_zero()
        if zero is None:
            raise sa_exc.InvalidRequestError(
                "Can't use filter_by when the first entity '%s' of a query "
                "is not a mapped class. Please use the filter method instead, "
                "or change the order of the entities in the query"
                % self._query_entity_zero()
            )

        clauses = [
            _entity_descriptor(zero, key) == value
            for key, value in kwargs.items()
        ]
        return self.filter(*clauses)

    @_generative(_no_statement_condition, _no_limit_offset)
    def order_by(self, *criterion):
        """Apply one or more ORDER BY criterion to the query and return
        the newly resulting :class:`_query.Query`.

        All existing ORDER BY settings can be suppressed by
        passing ``None`` - this will suppress any ordering configured
        on the :func:`.mapper` object using the deprecated
        :paramref:`.mapper.order_by` parameter.

        """

        if len(criterion) == 1:
            if criterion[0] is False:
                if "_order_by" in self.__dict__:
                    self._order_by = False
                return
            if criterion[0] is None:
                self._order_by = None
                return

        criterion = self._adapt_col_list(criterion)

        if self._order_by is False or self._order_by is None:
            self._order_by = criterion
        else:
            self._order_by = self._order_by + criterion

    @_generative(_no_statement_condition, _no_limit_offset)
    def group_by(self, *criterion):
        """Apply one or more GROUP BY criterion to the query and return
        the newly resulting :class:`_query.Query`.

        All existing GROUP BY settings can be suppressed by
        passing ``None`` - this will suppress any GROUP BY configured
        on mappers as well.

        .. versionadded:: 1.1 GROUP BY can be cancelled by passing
           ``None``, in the same way as ORDER BY.

        """

        if len(criterion) == 1:
            if criterion[0] is None:
                self._group_by = False
                return

        criterion = list(chain(*[_orm_columns(c) for c in criterion]))
        criterion = self._adapt_col_list(criterion)

        if self._group_by is False:
            self._group_by = criterion
        else:
            self._group_by = self._group_by + criterion

    @_generative(_no_statement_condition, _no_limit_offset)
    def having(self, criterion):
        r"""Apply a HAVING criterion to the query and return the
        newly resulting :class:`_query.Query`.

        :meth:`_query.Query.having` is used in conjunction with
        :meth:`_query.Query.group_by`.

        HAVING criterion makes it possible to use filters on aggregate
        functions like COUNT, SUM, AVG, MAX, and MIN, eg.::

            q = session.query(User.id).\
                        join(User.addresses).\
                        group_by(User.id).\
                        having(func.count(Address.id) > 2)

        """

        criterion = expression._expression_literal_as_text(criterion)

        if criterion is not None and not isinstance(
            criterion, sql.ClauseElement
        ):
            raise sa_exc.ArgumentError(
                "having() argument must be of type "
                "sqlalchemy.sql.ClauseElement or string"
            )

        criterion = self._adapt_clause(criterion, True, True)

        if self._having is not None:
            self._having = self._having & criterion
        else:
            self._having = criterion

    def _set_op(self, expr_fn, *q):
        return self._from_selectable(
            expr_fn(*([self] + list(q)))
        )._set_enable_single_crit(False)

    def union(self, *q):
        """Produce a UNION of this Query against one or more queries.

        e.g.::

            q1 = sess.query(SomeClass).filter(SomeClass.foo=='bar')
            q2 = sess.query(SomeClass).filter(SomeClass.bar=='foo')

            q3 = q1.union(q2)

        The method accepts multiple Query objects so as to control
        the level of nesting.  A series of ``union()`` calls such as::

            x.union(y).union(z).all()

        will nest on each ``union()``, and produces::

            SELECT * FROM (SELECT * FROM (SELECT * FROM X UNION
                            SELECT * FROM y) UNION SELECT * FROM Z)

        Whereas::

            x.union(y, z).all()

        produces::

            SELECT * FROM (SELECT * FROM X UNION SELECT * FROM y UNION
                            SELECT * FROM Z)

        Note that many database backends do not allow ORDER BY to
        be rendered on a query called within UNION, EXCEPT, etc.
        To disable all ORDER BY clauses including those configured
        on mappers, issue ``query.order_by(None)`` - the resulting
        :class:`_query.Query` object will not render ORDER BY within
        its SELECT statement.

        """
        return self._set_op(expression.union, *q)

    def union_all(self, *q):
        """Produce a UNION ALL of this Query against one or more queries.

        Works the same way as :meth:`~sqlalchemy.orm.query.Query.union`. See
        that method for usage examples.

        """
        return self._set_op(expression.union_all, *q)

    def intersect(self, *q):
        """Produce an INTERSECT of this Query against one or more queries.

        Works the same way as :meth:`~sqlalchemy.orm.query.Query.union`. See
        that method for usage examples.

        """
        return self._set_op(expression.intersect, *q)

    def intersect_all(self, *q):
        """Produce an INTERSECT ALL of this Query against one or more queries.

        Works the same way as :meth:`~sqlalchemy.orm.query.Query.union`. See
        that method for usage examples.

        """
        return self._set_op(expression.intersect_all, *q)

    def except_(self, *q):
        """Produce an EXCEPT of this Query against one or more queries.

        Works the same way as :meth:`~sqlalchemy.orm.query.Query.union`. See
        that method for usage examples.

        """
        return self._set_op(expression.except_, *q)

    def except_all(self, *q):
        """Produce an EXCEPT ALL of this Query against one or more queries.

        Works the same way as :meth:`~sqlalchemy.orm.query.Query.union`. See
        that method for usage examples.

        """
        return self._set_op(expression.except_all, *q)

    def join(self, *props, **kwargs):
        r"""Create a SQL JOIN against this :class:`_query.Query`
        object's criterion
        and apply generatively, returning the newly resulting
        :class:`_query.Query`.

        **Simple Relationship Joins**

        Consider a mapping between two classes ``User`` and ``Address``,
        with a relationship ``User.addresses`` representing a collection
        of ``Address`` objects associated with each ``User``.   The most
        common usage of :meth:`_query.Query.join`
        is to create a JOIN along this
        relationship, using the ``User.addresses`` attribute as an indicator
        for how this should occur::

            q = session.query(User).join(User.addresses)

        Where above, the call to :meth:`_query.Query.join` along
        ``User.addresses`` will result in SQL approximately equivalent to::

            SELECT user.id, user.name
            FROM user JOIN address ON user.id = address.user_id

        In the above example we refer to ``User.addresses`` as passed to
        :meth:`_query.Query.join` as the "on clause", that is, it indicates
        how the "ON" portion of the JOIN should be constructed.

        To construct a chain of joins, multiple :meth:`_query.Query.join`
        calls may be used.  The relationship-bound attribute implies both
        the left and right side of the join at once::

            q = session.query(User).\
                    join(User.orders).\
                    join(Order.items).\
                    join(Item.keywords)

        .. note:: as seen in the above example, **the order in which each
           call to the join() method occurs is important**.    Query would not,
           for example, know how to join correctly if we were to specify
           ``User``, then ``Item``, then ``Order``, in our chain of joins; in
           such a case, depending on the arguments passed, it may raise an
           error that it doesn't know how to join, or it may produce invalid
           SQL in which case the database will raise an error. In correct
           practice, the
           :meth:`_query.Query.join` method is invoked in such a way that lines
           up with how we would want the JOIN clauses in SQL to be
           rendered, and each call should represent a clear link from what
           precedes it.

        **Joins to a Target Entity or Selectable**

        A second form of :meth:`_query.Query.join` allows any mapped entity or
        core selectable construct as a target.   In this usage,
        :meth:`_query.Query.join` will attempt to create a JOIN along the
        natural foreign key relationship between two entities::

            q = session.query(User).join(Address)

        In the above calling form, :meth:`_query.Query.join` is called upon to
        create the "on clause" automatically for us.  This calling form will
        ultimately raise an error if either there are no foreign keys between
        the two entities, or if there are multiple foreign key linkages between
        the target entity and the entity or entities already present on the
        left side such that creating a join requires more information.  Note
        that when indicating a join to a target without any ON clause, ORM
        configured relationships are not taken into account.

        **Joins to a Target with an ON Clause**

        The third calling form allows both the target entity as well
        as the ON clause to be passed explicitly.    A example that includes
        a SQL expression as the ON clause is as follows::

            q = session.query(User).join(Address, User.id==Address.user_id)

        The above form may also use a relationship-bound attribute as the
        ON clause as well::

            q = session.query(User).join(Address, User.addresses)

        The above syntax can be useful for the case where we wish
        to join to an alias of a particular target entity.  If we wanted
        to join to ``Address`` twice, it could be achieved using two
        aliases set up using the :func:`~sqlalchemy.orm.aliased` function::

            a1 = aliased(Address)
            a2 = aliased(Address)

            q = session.query(User).\
                    join(a1, User.addresses).\
                    join(a2, User.addresses).\
                    filter(a1.email_address=='ed@foo.com').\
                    filter(a2.email_address=='ed@bar.com')

        The relationship-bound calling form can also specify a target entity
        using the :meth:`_orm.PropComparator.of_type` method; a query
        equivalent to the one above would be::

            a1 = aliased(Address)
            a2 = aliased(Address)

            q = session.query(User).\
                    join(User.addresses.of_type(a1)).\
                    join(User.addresses.of_type(a2)).\
                    filter(a1.email_address == 'ed@foo.com').\
                    filter(a2.email_address == 'ed@bar.com')

        **Joining to Tables and Subqueries**


        The target of a join may also be any table or SELECT statement,
        which may be related to a target entity or not.   Use the
        appropriate ``.subquery()`` method in order to make a subquery
        out of a query::

            subq = session.query(Address).\
                filter(Address.email_address == 'ed@foo.com').\
                subquery()


            q = session.query(User).join(
                subq, User.id == subq.c.user_id
            )

        Joining to a subquery in terms of a specific relationship and/or
        target entity may be achieved by linking the subquery to the
        entity using :func:`_orm.aliased`::

            subq = session.query(Address).\
                filter(Address.email_address == 'ed@foo.com').\
                subquery()

            address_subq = aliased(Address, subq)

            q = session.query(User).join(
                User.addresses.of_type(address_subq)
            )


        **Controlling what to Join From**

        In cases where the left side of the current state of
        :class:`_query.Query` is not in line with what we want to join from,
        the :meth:`_query.Query.select_from` method may be used::

            q = session.query(Address).select_from(User).\
                            join(User.addresses).\
                            filter(User.name == 'ed')

        Which will produce SQL similar to::

            SELECT address.* FROM user
                JOIN address ON user.id=address.user_id
                WHERE user.name = :name_1

        **Legacy Features of Query.join()**

        The :meth:`_query.Query.join` method currently supports several
        usage patterns and arguments that are considered to be legacy
        as of SQLAlchemy 1.3.   A deprecation path will follow
        in the 1.4 series for the following features:


        * Joining on relationship names rather than attributes::

            session.query(User).join("addresses")

          **Why it's legacy**: the string name does not provide enough context
          for :meth:`_query.Query.join` to always know what is desired,
          notably in that there is no indication of what the left side
          of the join should be.  This gives rise to flags like
          ``from_joinpoint`` as well as the ability to place several
          join clauses in a single :meth:`_query.Query.join` call
          which don't solve the problem fully while also
          adding new calling styles that are unnecessary and expensive to
          accommodate internally.

          **Modern calling pattern**:  Use the actual relationship,
          e.g. ``User.addresses`` in the above case::

              session.query(User).join(User.addresses)

        * Automatic aliasing with the ``aliased=True`` flag::

            session.query(Node).join(Node.children, aliased=True).\
                filter(Node.name == 'some name')

          **Why it's legacy**:  the automatic aliasing feature of
          :class:`_query.Query` is intensely complicated, both in its internal
          implementation as well as in its observed behavior, and is almost
          never used.  It is difficult to know upon inspection where and when
          its aliasing of a target entity, ``Node`` in the above case, will be
          applied and when it won't, and additionally the feature has to use
          very elaborate heuristics to achieve this implicit behavior.

          **Modern calling pattern**: Use the :func:`_orm.aliased` construct
          explicitly::

            from sqlalchemy.orm import aliased

            n1 = aliased(Node)

            session.query(Node).join(Node.children.of_type(n1)).\
                filter(n1.name == 'some name')

        * Multiple joins in one call::

            session.query(User).join("orders", "items")

            session.query(User).join(User.orders, Order.items)

            session.query(User).join(
                (Order, User.orders),
                (Item, Item.order_id == Order.id)
            )

            # ... and several more forms actually

          **Why it's legacy**: being able to chain multiple ON clauses in one
          call to :meth:`_query.Query.join` is yet another attempt to solve
          the problem of being able to specify what entity to join from,
          and is the source of a large variety of potential calling patterns
          that are internally expensive and complicated to parse and
          accommodate.

          **Modern calling pattern**:  Use relationship-bound attributes
          or SQL-oriented ON clauses within separate calls, so that
          each call to :meth:`_query.Query.join` knows what the left
          side should be::

            session.query(User).join(User.orders).join(
                Item, Item.order_id == Order.id)


        :param \*props: Incoming arguments for :meth:`_query.Query.join`,
         the props collection in modern use should be considered to be a  one
         or two argument form, either as a single "target" entity or ORM
         attribute-bound relationship, or as a target entity plus an "on
         clause" which  may be a SQL expression or ORM attribute-bound
         relationship.

        :param isouter=False: If True, the join used will be a left outer join,
         just as if the :meth:`_query.Query.outerjoin` method were called.

        :param full=False: render FULL OUTER JOIN; implies ``isouter``.

         .. versionadded:: 1.1

        :param from_joinpoint=False: When using ``aliased=True``, a setting
         of True here will cause the join to be from the most recent
         joined target, rather than starting back from the original
         FROM clauses of the query.

         .. note:: This flag is considered legacy.

        :param aliased=False: If True, indicate that the JOIN target should be
         anonymously aliased.  Subsequent calls to :meth:`_query.Query.filter`
         and similar will adapt the incoming criterion to the target
         alias, until :meth:`_query.Query.reset_joinpoint` is called.

         .. note:: This flag is considered legacy.

        .. seealso::

            :ref:`ormtutorial_joins` in the ORM tutorial.

            :ref:`inheritance_toplevel` for details on how
            :meth:`_query.Query.join` is used for inheritance relationships.

            :func:`_orm.join` - a standalone ORM-level join function,
            used internally by :meth:`_query.Query.join`, which in previous
            SQLAlchemy versions was the primary ORM-level joining interface.

        """
        aliased, from_joinpoint, isouter, full = (
            kwargs.pop("aliased", False),
            kwargs.pop("from_joinpoint", False),
            kwargs.pop("isouter", False),
            kwargs.pop("full", False),
        )
        if kwargs:
            raise TypeError(
                "unknown arguments: %s" % ", ".join(sorted(kwargs))
            )
        return self._join(
            props,
            outerjoin=isouter,
            full=full,
            create_aliases=aliased,
            from_joinpoint=from_joinpoint,
        )

    def outerjoin(self, *props, **kwargs):
        """Create a left outer join against this :class:`_query.Query`
        object's criterion and apply generatively, returning the newly
        resulting :class:`_query.Query`.

        Usage is the same as the :meth:`_query.Query.join` method.

        """
        aliased, from_joinpoint, full = (
            kwargs.pop("aliased", False),
            kwargs.pop("from_joinpoint", False),
            kwargs.pop("full", False),
        )
        if kwargs:
            raise TypeError(
                "unknown arguments: %s" % ", ".join(sorted(kwargs))
            )
        return self._join(
            props,
            outerjoin=True,
            full=full,
            create_aliases=aliased,
            from_joinpoint=from_joinpoint,
        )

    def _update_joinpoint(self, jp):
        self._joinpoint = jp
        # copy backwards to the root of the _joinpath
        # dict, so that no existing dict in the path is mutated
        while "prev" in jp:
            f, prev = jp["prev"]
            prev = prev.copy()
            prev[f] = jp.copy()
            jp["prev"] = (f, prev)
            jp = prev
        self._joinpath = jp

    @_generative(_no_statement_condition, _no_limit_offset)
    def _join(self, keys, outerjoin, full, create_aliases, from_joinpoint):
        """Consumes arguments from :meth:`_query.Query.join` or
        :meth:`_query.Query.outerjoin`, places them into a
        consistent format with which to form the actual JOIN constructs.

        """

        if not from_joinpoint:
            self._reset_joinpoint()

        if (
            len(keys) == 2
            and isinstance(
                keys[0], (expression.FromClause, type, AliasedClass)
            )
            and isinstance(
                keys[1],
                (str, expression.ClauseElement, interfaces.PropComparator),
            )
        ):
            # detect 2-arg form of join and
            # convert to a tuple.
            keys = (keys,)

        # Query.join() accepts a list of join paths all at once.
        # step one is to iterate through these paths and determine the
        # intent of each path individually.  as we encounter a path token,
        # we add a new ORMJoin construct to the self._from_obj tuple,
        # either by adding a new element to it, or by replacing an existing
        # element with a new ORMJoin.
        keylist = util.to_list(keys)
        for idx, arg1 in enumerate(keylist):
            if isinstance(arg1, tuple):
                # "tuple" form of join, multiple
                # tuples are accepted as well.   The simpler
                # "2-arg" form is preferred.
                arg1, arg2 = arg1
            else:
                arg2 = None

            # determine onclause/right_entity.  there
            # is a little bit of legacy behavior still at work here
            # which means they might be in either order.
            if isinstance(
                arg1, (interfaces.PropComparator, util.string_types)
            ):
                right, onclause = arg2, arg1
            else:
                right, onclause = arg1, arg2

            if onclause is None:
                r_info = inspect(right)
                if not r_info.is_selectable and not hasattr(r_info, "mapper"):
                    raise sa_exc.ArgumentError(
                        "Expected mapped entity or "
                        "selectable/table as join target"
                    )

            if isinstance(onclause, interfaces.PropComparator):
                of_type = getattr(onclause, "_of_type", None)
            else:
                of_type = None

            if isinstance(onclause, util.string_types):
                # string given, e.g. query(Foo).join("bar").
                # we look to the left entity or what we last joined
                # towards
                onclause = _entity_descriptor(self._joinpoint_zero(), onclause)

            # check for q.join(Class.propname, from_joinpoint=True)
            # and Class corresponds at the mapper level to the current
            # joinpoint.  this match intentionally looks for a non-aliased
            # class-bound descriptor as the onclause and if it matches the
            # current joinpoint at the mapper level, it's used.  This
            # is a very old use case that is intended to make it easier
            # to work with the aliased=True flag, which is also something
            # that probably shouldn't exist on join() due to its high
            # complexity/usefulness ratio
            elif from_joinpoint and isinstance(
                onclause, interfaces.PropComparator
            ):
                jp0 = self._joinpoint_zero()
                info = inspect(jp0)

                if getattr(info, "mapper", None) is onclause._parententity:
                    onclause = _entity_descriptor(jp0, onclause.key)

            if isinstance(onclause, interfaces.PropComparator):
                # descriptor/property given (or determined); this tells
                # us explicitly what the expected "left" side of the join is.
                if right is None:
                    if of_type:
                        right = of_type
                    else:
                        right = onclause.property

                        try:
                            right = right.entity
                        except AttributeError as err:
                            util.raise_(
                                sa_exc.ArgumentError(
                                    "Join target %s does not refer to a "
                                    "mapped entity" % right
                                ),
                                replace_context=err,
                            )

                left = onclause._parententity

                alias = self._polymorphic_adapters.get(left, None)

                # could be None or could be ColumnAdapter also
                if isinstance(alias, ORMAdapter) and alias.mapper.isa(left):
                    left = alias.aliased_class
                    onclause = getattr(left, onclause.key)

                prop = onclause.property
                if not isinstance(onclause, attributes.QueryableAttribute):
                    onclause = prop

                if not create_aliases:
                    # check for this path already present.
                    # don't render in that case.
                    edge = (left, right, prop.key)
                    if edge in self._joinpoint:
                        # The child's prev reference might be stale --
                        # it could point to a parent older than the
                        # current joinpoint.  If this is the case,
                        # then we need to update it and then fix the
                        # tree's spine with _update_joinpoint.  Copy
                        # and then mutate the child, which might be
                        # shared by a different query object.
                        jp = self._joinpoint[edge].copy()
                        jp["prev"] = (edge, self._joinpoint)
                        self._update_joinpoint(jp)

                        # warn only on the last element of the list
                        if idx == len(keylist) - 1:
                            util.warn(
                                "Pathed join target %s has already "
                                "been joined to; skipping" % prop
                            )
                        continue
            else:
                # no descriptor/property given; we will need to figure out
                # what the effective "left" side is
                prop = left = None

            # figure out the final "left" and "right" sides and create an
            # ORMJoin to add to our _from_obj tuple
            self._join_left_to_right(
                left, right, onclause, prop, create_aliases, outerjoin, full
            )

    def _join_left_to_right(
        self, left, right, onclause, prop, create_aliases, outerjoin, full
    ):
        """Given raw "left", "right", "onclause" parameters consumed from
        a particular key within _join(), add a real ORMJoin object to
        our _from_obj list (or augment an existing one)

        """

        self._polymorphic_adapters = self._polymorphic_adapters.copy()

        if left is None:
            # left not given (e.g. no relationship object/name specified)
            # figure out the best "left" side based on our existing froms /
            # entities
            assert prop is None
            (
                left,
                replace_from_obj_index,
                use_entity_index,
            ) = self._join_determine_implicit_left_side(left, right, onclause)
        else:
            # left is given via a relationship/name.  Determine where in our
            # "froms" list it should be spliced/appended as well as what
            # existing entity it corresponds to.
            assert prop is not None
            (
                replace_from_obj_index,
                use_entity_index,
            ) = self._join_place_explicit_left_side(left)

        if left is right and not create_aliases:
            raise sa_exc.InvalidRequestError(
                "Can't construct a join from %s to %s, they "
                "are the same entity" % (left, right)
            )

        # the right side as given often needs to be adapted.  additionally
        # a lot of things can be wrong with it.  handle all that and
        # get back the new effective "right" side
        r_info, right, onclause = self._join_check_and_adapt_right_side(
            left, right, onclause, prop, create_aliases
        )

        if replace_from_obj_index is not None:
            # splice into an existing element in the
            # self._from_obj list
            left_clause = self._from_obj[replace_from_obj_index]

            self._from_obj = (
                self._from_obj[:replace_from_obj_index]
                + (
                    orm_join(
                        left_clause,
                        right,
                        onclause,
                        isouter=outerjoin,
                        full=full,
                    ),
                )
                + self._from_obj[replace_from_obj_index + 1 :]
            )
        else:
            # add a new element to the self._from_obj list

            if use_entity_index is not None:
                # why doesn't this work as .entity_zero_or_selectable?
                left_clause = self._entities[use_entity_index].selectable
            else:
                left_clause = left

            self._from_obj = self._from_obj + (
                orm_join(
                    left_clause, right, onclause, isouter=outerjoin, full=full
                ),
            )

    def _join_determine_implicit_left_side(self, left, right, onclause):
        """When join conditions don't express the left side explicitly,
        determine if an existing FROM or entity in this query
        can serve as the left hand side.

        """

        # when we are here, it means join() was called without an ORM-
        # specific way of telling us what the "left" side is, e.g.:
        #
        # join(RightEntity)
        #
        # or
        #
        # join(RightEntity, RightEntity.foo == LeftEntity.bar)
        #

        r_info = inspect(right)

        replace_from_obj_index = use_entity_index = None

        if self._from_obj:
            # we have a list of FROMs already.  So by definition this
            # join has to connect to one of those FROMs.

            indexes = sql_util.find_left_clause_to_join_from(
                self._from_obj, r_info.selectable, onclause
            )

            if len(indexes) == 1:
                replace_from_obj_index = indexes[0]
                left = self._from_obj[replace_from_obj_index]
            elif len(indexes) > 1:
                raise sa_exc.InvalidRequestError(
                    "Can't determine which FROM clause to join "
                    "from, there are multiple FROMS which can "
                    "join to this entity. Please use the .select_from() "
                    "method to establish an explicit left side, as well as "
                    "providing an explcit ON clause if not present already to "
                    "help resolve the ambiguity."
                )
            else:
                raise sa_exc.InvalidRequestError(
                    "Don't know how to join to %r. "
                    "Please use the .select_from() "
                    "method to establish an explicit left side, as well as "
                    "providing an explcit ON clause if not present already to "
                    "help resolve the ambiguity." % (right,)
                )

        elif self._entities:
            # we have no explicit FROMs, so the implicit left has to
            # come from our list of entities.

            potential = {}
            for entity_index, ent in enumerate(self._entities):
                entity = ent.entity_zero_or_selectable
                if entity is None:
                    continue
                ent_info = inspect(entity)
                if ent_info is r_info:  # left and right are the same, skip
                    continue

                # by using a dictionary with the selectables as keys this
                # de-duplicates those selectables as occurs when the query is
                # against a series of columns from the same selectable
                if isinstance(ent, _MapperEntity):
                    potential[ent.selectable] = (entity_index, entity)
                else:
                    potential[ent_info.selectable] = (None, entity)

            all_clauses = list(potential.keys())
            indexes = sql_util.find_left_clause_to_join_from(
                all_clauses, r_info.selectable, onclause
            )

            if len(indexes) == 1:
                use_entity_index, left = potential[all_clauses[indexes[0]]]
            elif len(indexes) > 1:
                raise sa_exc.InvalidRequestError(
                    "Can't determine which FROM clause to join "
                    "from, there are multiple FROMS which can "
                    "join to this entity. Please use the .select_from() "
                    "method to establish an explicit left side, as well as "
                    "providing an explcit ON clause if not present already to "
                    "help resolve the ambiguity."
                )
            else:
                raise sa_exc.InvalidRequestError(
                    "Don't know how to join to %r. "
                    "Please use the .select_from() "
                    "method to establish an explicit left side, as well as "
                    "providing an explcit ON clause if not present already to "
                    "help resolve the ambiguity." % (right,)
                )
        else:
            raise sa_exc.InvalidRequestError(
                "No entities to join from; please use "
                "select_from() to establish the left "
                "entity/selectable of this join"
            )

        return left, replace_from_obj_index, use_entity_index

    def _join_place_explicit_left_side(self, left):
        """When join conditions express a left side explicitly, determine
        where in our existing list of FROM clauses we should join towards,
        or if we need to make a new join, and if so is it from one of our
        existing entities.

        """

        # when we are here, it means join() was called with an indicator
        # as to an exact left side, which means a path to a
        # RelationshipProperty was given, e.g.:
        #
        # join(RightEntity, LeftEntity.right)
        #
        # or
        #
        # join(LeftEntity.right)
        #
        # as well as string forms:
        #
        # join(RightEntity, "right")
        #
        # etc.
        #

        replace_from_obj_index = use_entity_index = None

        l_info = inspect(left)
        if self._from_obj:
            indexes = sql_util.find_left_clause_that_matches_given(
                self._from_obj, l_info.selectable
            )

            if len(indexes) > 1:
                raise sa_exc.InvalidRequestError(
                    "Can't identify which entity in which to assign the "
                    "left side of this join.   Please use a more specific "
                    "ON clause."
                )

            # have an index, means the left side is already present in
            # an existing FROM in the self._from_obj tuple
            if indexes:
                replace_from_obj_index = indexes[0]

            # no index, means we need to add a new element to the
            # self._from_obj tuple

        # no from element present, so we will have to add to the
        # self._from_obj tuple.  Determine if this left side matches up
        # with existing mapper entities, in which case we want to apply the
        # aliasing / adaptation rules present on that entity if any
        if (
            replace_from_obj_index is None
            and self._entities
            and hasattr(l_info, "mapper")
        ):
            for idx, ent in enumerate(self._entities):
                # TODO: should we be checking for multiple mapper entities
                # matching?
                if isinstance(ent, _MapperEntity) and ent.corresponds_to(left):
                    use_entity_index = idx
                    break

        return replace_from_obj_index, use_entity_index

    def _join_check_and_adapt_right_side(
        self, left, right, onclause, prop, create_aliases
    ):
        """Transform the "right" side of the join as well as the onclause
        according to polymorphic mapping translations, aliasing on the query
        or on the join, special cases where the right and left side have
        overlapping tables.

        """

        l_info = inspect(left)
        r_info = inspect(right)

        overlap = False
        if not create_aliases:
            right_mapper = getattr(r_info, "mapper", None)
            # if the target is a joined inheritance mapping,
            # be more liberal about auto-aliasing.
            if right_mapper and (
                right_mapper.with_polymorphic
                or isinstance(right_mapper.persist_selectable, expression.Join)
            ):
                for from_obj in self._from_obj or [l_info.selectable]:
                    if sql_util.selectables_overlap(
                        l_info.selectable, from_obj
                    ) and sql_util.selectables_overlap(
                        from_obj, r_info.selectable
                    ):
                        overlap = True
                        break

        if (
            overlap or not create_aliases
        ) and l_info.selectable is r_info.selectable:
            raise sa_exc.InvalidRequestError(
                "Can't join table/selectable '%s' to itself"
                % l_info.selectable
            )

        right_mapper, right_selectable, right_is_aliased = (
            getattr(r_info, "mapper", None),
            r_info.selectable,
            getattr(r_info, "is_aliased_class", False),
        )

        if (
            right_mapper
            and prop
            and not right_mapper.common_parent(prop.mapper)
        ):
            raise sa_exc.InvalidRequestError(
                "Join target %s does not correspond to "
                "the right side of join condition %s" % (right, onclause)
            )

        # _join_entities is used as a hint for single-table inheritance
        # purposes at the moment
        if hasattr(r_info, "mapper"):
            self._join_entities += (r_info,)

        need_adapter = False

        # test for joining to an unmapped selectable as the target
        if r_info.is_clause_element:

            if prop:
                right_mapper = prop.mapper

            if right_selectable._is_lateral:
                # orm_only is disabled to suit the case where we have to
                # adapt an explicit correlate(Entity) - the select() loses
                # the ORM-ness in this case right now, ideally it would not
                right = self._adapt_clause(right, True, False)

            elif prop:
                # joining to selectable with a mapper property given
                # as the ON clause
                if not right_selectable.is_derived_from(
                    right_mapper.persist_selectable
                ):
                    raise sa_exc.InvalidRequestError(
                        "Selectable '%s' is not derived from '%s'"
                        % (
                            right_selectable.description,
                            right_mapper.persist_selectable.description,
                        )
                    )

                # if the destination selectable is a plain select(),
                # turn it into an alias().
                if isinstance(right_selectable, expression.SelectBase):
                    right_selectable = right_selectable.alias()
                    need_adapter = True

                # make the right hand side target into an ORM entity
                right = aliased(right_mapper, right_selectable)
            elif create_aliases:
                # it *could* work, but it doesn't right now and I'd rather
                # get rid of aliased=True completely
                raise sa_exc.InvalidRequestError(
                    "The aliased=True parameter on query.join() only works "
                    "with an ORM entity, not a plain selectable, as the "
                    "target."
                )

        aliased_entity = (
            right_mapper
            and not right_is_aliased
            and (
                right_mapper.with_polymorphic
                and isinstance(
                    right_mapper._with_polymorphic_selectable, expression.Alias
                )
                or overlap
                # test for overlap:
                # orm/inheritance/relationships.py
                # SelfReferentialM2MTest
            )
        )

        if not need_adapter and (create_aliases or aliased_entity):
            right = aliased(right, flat=True)
            need_adapter = True

        if need_adapter:
            assert right_mapper

            # if an alias() of the right side was generated,
            # apply an adapter to all subsequent filter() calls
            # until reset_joinpoint() is called.
            adapter = ORMAdapter(
                right, equivalents=right_mapper._equivalent_columns
            )
            # current adapter takes highest precedence
            self._filter_aliases = (adapter,) + self._filter_aliases

            # if an alias() on the right side was generated,
            # which is intended to wrap a the right side in a subquery,
            # ensure that columns retrieved from this target in the result
            # set are also adapted.
            if not create_aliases:
                self._mapper_loads_polymorphically_with(right_mapper, adapter)

        # if the onclause is a ClauseElement, adapt it with any
        # adapters that are in place right now
        if isinstance(onclause, expression.ClauseElement):
            onclause = self._adapt_clause(onclause, True, True)

        # if joining on a MapperProperty path,
        # track the path to prevent redundant joins
        if not create_aliases and prop:
            self._update_joinpoint(
                {
                    "_joinpoint_entity": right,
                    "prev": ((left, right, prop.key), self._joinpoint),
                }
            )
        else:
            self._joinpoint = {"_joinpoint_entity": right}

        return right, inspect(right), onclause

    def _reset_joinpoint(self):
        self._joinpoint = self._joinpath
        self._filter_aliases = ()

    @_generative(_no_statement_condition)
    def reset_joinpoint(self):
        """Return a new :class:`_query.Query`, where the "join point" has
        been reset back to the base FROM entities of the query.

        This method is usually used in conjunction with the
        ``aliased=True`` feature of the :meth:`_query.Query.join`
        method.  See the example in :meth:`_query.Query.join` for how
        this is used.

        """
        self._reset_joinpoint()

    @_generative(_no_clauseelement_condition)
    def select_from(self, *from_obj):
        r"""Set the FROM clause of this :class:`_query.Query` explicitly.

        :meth:`_query.Query.select_from` is often used in conjunction with
        :meth:`_query.Query.join` in order to control which entity is selected
        from on the "left" side of the join.

        The entity or selectable object here effectively replaces the
        "left edge" of any calls to :meth:`_query.Query.join`, when no
        joinpoint is otherwise established - usually, the default "join
        point" is the leftmost entity in the :class:`_query.Query` object's
        list of entities to be selected.

        A typical example::

            q = session.query(Address).select_from(User).\
                join(User.addresses).\
                filter(User.name == 'ed')

        Which produces SQL equivalent to::

            SELECT address.* FROM user
            JOIN address ON user.id=address.user_id
            WHERE user.name = :name_1

        :param \*from_obj: collection of one or more entities to apply
         to the FROM clause.  Entities can be mapped classes,
         :class:`.AliasedClass` objects, :class:`_orm.Mapper` objects
         as well as core :class:`_expression.FromClause`
         elements like subqueries.

        .. versionchanged:: 0.9
            This method no longer applies the given FROM object
            to be the selectable from which matching entities
            select from; the :meth:`.select_entity_from` method
            now accomplishes this.  See that method for a description
            of this behavior.

        .. seealso::

            :meth:`_query.Query.join`

            :meth:`_query.Query.select_entity_from`

        """

        self._set_select_from(from_obj, False)

    @_generative(_no_clauseelement_condition)
    def select_entity_from(self, from_obj):
        r"""Set the FROM clause of this :class:`_query.Query` to a
        core selectable, applying it as a replacement FROM clause
        for corresponding mapped entities.

        The :meth:`_query.Query.select_entity_from`
        method supplies an alternative
        approach to the use case of applying an :func:`.aliased` construct
        explicitly throughout a query.  Instead of referring to the
        :func:`.aliased` construct explicitly,
        :meth:`_query.Query.select_entity_from` automatically *adapts* all
        occurrences of the entity to the target selectable.

        Given a case for :func:`.aliased` such as selecting ``User``
        objects from a SELECT statement::

            select_stmt = select([User]).where(User.id == 7)
            user_alias = aliased(User, select_stmt)

            q = session.query(user_alias).\
                filter(user_alias.name == 'ed')

        Above, we apply the ``user_alias`` object explicitly throughout the
        query.  When it's not feasible for ``user_alias`` to be referenced
        explicitly in many places, :meth:`_query.Query.select_entity_from`
        may be
        used at the start of the query to adapt the existing ``User`` entity::

            q = session.query(User).\
                select_entity_from(select_stmt).\
                filter(User.name == 'ed')

        Above, the generated SQL will show that the ``User`` entity is
        adapted to our statement, even in the case of the WHERE clause:

        .. sourcecode:: sql

            SELECT anon_1.id AS anon_1_id, anon_1.name AS anon_1_name
            FROM (SELECT "user".id AS id, "user".name AS name
            FROM "user"
            WHERE "user".id = :id_1) AS anon_1
            WHERE anon_1.name = :name_1

        The :meth:`_query.Query.select_entity_from` method is similar to the
        :meth:`_query.Query.select_from` method,
        in that it sets the FROM clause
        of the query.  The difference is that it additionally applies
        adaptation to the other parts of the query that refer to the
        primary entity.  If above we had used :meth:`_query.Query.select_from`
        instead, the SQL generated would have been:

        .. sourcecode:: sql

            -- uses plain select_from(), not select_entity_from()
            SELECT "user".id AS user_id, "user".name AS user_name
            FROM "user", (SELECT "user".id AS id, "user".name AS name
            FROM "user"
            WHERE "user".id = :id_1) AS anon_1
            WHERE "user".name = :name_1

        To supply textual SQL to the :meth:`_query.Query.select_entity_from`
        method,
        we can make use of the :func:`_expression.text` construct.  However,
        the
        :func:`_expression.text`
        construct needs to be aligned with the columns of our
        entity, which is achieved by making use of the
        :meth:`_expression.TextClause.columns` method::

            text_stmt = text("select id, name from user").columns(
                User.id, User.name)
            q = session.query(User).select_entity_from(text_stmt)

        :meth:`_query.Query.select_entity_from` itself accepts an
        :func:`.aliased`
        object, so that the special options of :func:`.aliased` such as
        :paramref:`.aliased.adapt_on_names` may be used within the
        scope of the :meth:`_query.Query.select_entity_from`
        method's adaptation
        services.  Suppose
        a view ``user_view`` also returns rows from ``user``.    If
        we reflect this view into a :class:`_schema.Table`, this view has no
        relationship to the :class:`_schema.Table` to which we are mapped,
        however
        we can use name matching to select from it::

            user_view = Table('user_view', metadata,
                              autoload_with=engine)
            user_view_alias = aliased(
                User, user_view, adapt_on_names=True)
            q = session.query(User).\
                select_entity_from(user_view_alias).\
                order_by(User.name)

        .. versionchanged:: 1.1.7 The :meth:`_query.Query.select_entity_from`
           method now accepts an :func:`.aliased` object as an alternative
           to a :class:`_expression.FromClause` object.

        :param from_obj: a :class:`_expression.FromClause`
         object that will replace
         the FROM clause of this :class:`_query.Query`.
         It also may be an instance
         of :func:`.aliased`.



        .. seealso::

            :meth:`_query.Query.select_from`

        """

        self._set_select_from([from_obj], True)

    def __getitem__(self, item):
        if isinstance(item, slice):
            start, stop, step = util.decode_slice(item)

            if (
                isinstance(stop, int)
                and isinstance(start, int)
                and stop - start <= 0
            ):
                return []

            # perhaps we should execute a count() here so that we
            # can still use LIMIT/OFFSET ?
            elif (isinstance(start, int) and start < 0) or (
                isinstance(stop, int) and stop < 0
            ):
                return list(self)[item]

            res = self.slice(start, stop)
            if step is not None:
                return list(res)[None : None : item.step]
            else:
                return list(res)
        else:
            if item == -1:
                return list(self)[-1]
            else:
                return list(self[item : item + 1])[0]

    @_generative(_no_statement_condition)
    def slice(self, start, stop):
        """Computes the "slice" of the :class:`_query.Query` represented by
        the given indices and returns the resulting :class:`_query.Query`.

        The start and stop indices behave like the argument to Python's
        built-in :func:`range` function. This method provides an
        alternative to using ``LIMIT``/``OFFSET`` to get a slice of the
        query.

        For example, ::

            session.query(User).order_by(User.id).slice(1, 3)

        renders as

        .. sourcecode:: sql

           SELECT users.id AS users_id,
                  users.name AS users_name
           FROM users ORDER BY users.id
           LIMIT ? OFFSET ?
           (2, 1)

        .. seealso::

           :meth:`_query.Query.limit`

           :meth:`_query.Query.offset`

        """
        if start is not None and stop is not None:
            self._offset = self._offset if self._offset is not None else 0
            if start != 0:
                self._offset += start
            self._limit = stop - start
        elif start is None and stop is not None:
            self._limit = stop
        elif start is not None and stop is None:
            self._offset = self._offset if self._offset is not None else 0
            if start != 0:
                self._offset += start

        if isinstance(self._offset, int) and self._offset == 0:
            self._offset = None

    @_generative(_no_statement_condition)
    def limit(self, limit):
        """Apply a ``LIMIT`` to the query and return the newly resulting
        :class:`_query.Query`.

        """
        self._limit = limit

    @_generative(_no_statement_condition)
    def offset(self, offset):
        """Apply an ``OFFSET`` to the query and return the newly resulting
        :class:`_query.Query`.

        """
        self._offset = offset

    @_generative(_no_statement_condition)
    def distinct(self, *expr):
        r"""Apply a ``DISTINCT`` to the query and return the newly resulting
        :class:`_query.Query`.


        .. note::

            The :meth:`.distinct` call includes logic that will automatically
            add columns from the ORDER BY of the query to the columns
            clause of the SELECT statement, to satisfy the common need
            of the database backend that ORDER BY columns be part of the
            SELECT list when DISTINCT is used.   These columns *are not*
            added to the list of columns actually fetched by the
            :class:`_query.Query`, however, so would not affect results.
            The columns are passed through when using the
            :attr:`_query.Query.statement` accessor, however.

        :param \*expr: optional column expressions.  When present,
         the PostgreSQL dialect will render a ``DISTINCT ON (<expressions>)``
         construct.

        """
        if not expr:
            self._distinct = True
        else:
            expr = self._adapt_col_list(expr)
            if isinstance(self._distinct, list):
                self._distinct += expr
            else:
                self._distinct = expr

    @_generative()
    def prefix_with(self, *prefixes):
        r"""Apply the prefixes to the query and return the newly resulting
        :class:`_query.Query`.

        :param \*prefixes: optional prefixes, typically strings,
         not using any commas.   In particular is useful for MySQL keywords
         and optimizer hints:

        e.g.::

            query = sess.query(User.name).\
                prefix_with('HIGH_PRIORITY').\
                prefix_with('SQL_SMALL_RESULT', 'ALL').\
                prefix_with('/*+ BKA(user) */')

        Would render::

            SELECT HIGH_PRIORITY SQL_SMALL_RESULT ALL /*+ BKA(user) */
            users.name AS users_name FROM users

        .. seealso::

            :meth:`_expression.HasPrefixes.prefix_with`

        """
        if self._prefixes:
            self._prefixes += prefixes
        else:
            self._prefixes = prefixes

    @_generative()
    def suffix_with(self, *suffixes):
        r"""Apply the suffix to the query and return the newly resulting
        :class:`_query.Query`.

        :param \*suffixes: optional suffixes, typically strings,
         not using any commas.

        .. versionadded:: 1.0.0

        .. seealso::

            :meth:`_query.Query.prefix_with`

            :meth:`_expression.HasSuffixes.suffix_with`

        """
        if self._suffixes:
            self._suffixes += suffixes
        else:
            self._suffixes = suffixes

    def all(self):
        """Return the results represented by this :class:`_query.Query`
        as a list.

        This results in an execution of the underlying SQL statement.

        .. warning::  The :class:`_query.Query` object,
           when asked to return either
           a sequence or iterator that consists of full ORM-mapped entities,
           will **deduplicate entries based on primary key**.  See the FAQ for
           more details.

            .. seealso::

                :ref:`faq_query_deduplicating`
        """
        return list(self)

    @_generative(_no_clauseelement_condition)
    def from_statement(self, statement):
        """Execute the given SELECT statement and return results.

        This method bypasses all internal statement compilation, and the
        statement is executed without modification.

        The statement is typically either a :func:`_expression.text`
        or :func:`_expression.select` construct, and should return the set
        of columns
        appropriate to the entity class represented by this
        :class:`_query.Query`.

        .. seealso::

            :ref:`orm_tutorial_literal_sql` - usage examples in the
            ORM tutorial

        """
        statement = expression._expression_literal_as_text(statement)

        if not isinstance(
            statement, (expression.TextClause, expression.SelectBase)
        ):
            raise sa_exc.ArgumentError(
                "from_statement accepts text(), select(), "
                "and union() objects only."
            )

        self._statement = statement

    def first(self):
        """Return the first result of this ``Query`` or
        None if the result doesn't contain any row.

        first() applies a limit of one within the generated SQL, so that
        only one primary entity row is generated on the server side
        (note this may consist of multiple result rows if join-loaded
        collections are present).

        Calling :meth:`_query.Query.first`
        results in an execution of the underlying
        query.

        .. seealso::

            :meth:`_query.Query.one`

            :meth:`_query.Query.one_or_none`

        """
        if self._statement is not None:
            ret = list(self)[0:1]
        else:
            ret = list(self[0:1])
        if len(ret) > 0:
            return ret[0]
        else:
            return None

    def one_or_none(self):
        """Return at most one result or raise an exception.

        Returns ``None`` if the query selects
        no rows.  Raises ``sqlalchemy.orm.exc.MultipleResultsFound``
        if multiple object identities are returned, or if multiple
        rows are returned for a query that returns only scalar values
        as opposed to full identity-mapped entities.

        Calling :meth:`_query.Query.one_or_none`
        results in an execution of the
        underlying query.

        .. versionadded:: 1.0.9

            Added :meth:`_query.Query.one_or_none`

        .. seealso::

            :meth:`_query.Query.first`

            :meth:`_query.Query.one`

        """
        ret = list(self)

        l = len(ret)
        if l == 1:
            return ret[0]
        elif l == 0:
            return None
        else:
            raise orm_exc.MultipleResultsFound(
                "Multiple rows were found for one_or_none()"
            )

    def one(self):
        """Return exactly one result or raise an exception.

        Raises ``sqlalchemy.orm.exc.NoResultFound`` if the query selects
        no rows.  Raises ``sqlalchemy.orm.exc.MultipleResultsFound``
        if multiple object identities are returned, or if multiple
        rows are returned for a query that returns only scalar values
        as opposed to full identity-mapped entities.

        Calling :meth:`.one` results in an execution of the underlying query.

        .. seealso::

            :meth:`_query.Query.first`

            :meth:`_query.Query.one_or_none`

        """
        try:
            ret = self.one_or_none()
        except orm_exc.MultipleResultsFound as err:
            util.raise_(
                orm_exc.MultipleResultsFound(
                    "Multiple rows were found for one()"
                ),
                replace_context=err,
            )
        else:
            if ret is None:
                raise orm_exc.NoResultFound("No row was found for one()")
            return ret

    def scalar(self):
        """Return the first element of the first result or None
        if no rows present.  If multiple rows are returned,
        raises MultipleResultsFound.

          >>> session.query(Item).scalar()
          <Item>
          >>> session.query(Item.id).scalar()
          1
          >>> session.query(Item.id).filter(Item.id < 0).scalar()
          None
          >>> session.query(Item.id, Item.name).scalar()
          1
          >>> session.query(func.count(Parent.id)).scalar()
          20

        This results in an execution of the underlying query.

        """
        try:
            ret = self.one()
            if not isinstance(ret, tuple):
                return ret
            return ret[0]
        except orm_exc.NoResultFound:
            return None

    def __iter__(self):
        context = self._compile_context()
        context.statement.use_labels = True
        if self._autoflush and not self._populate_existing:
            self.session._autoflush()
        return self._execute_and_instances(context)

    def __str__(self):
        context = self._compile_context()
        try:
            bind = (
                self._get_bind_args(context, self.session.get_bind)
                if self.session
                else None
            )
        except sa_exc.UnboundExecutionError:
            bind = None
        return str(context.statement.compile(bind))

    def _connection_from_session(self, **kw):
        conn = self.session.connection(**kw)
        if self._execution_options:
            conn = conn.execution_options(**self._execution_options)
        return conn

    def _execute_and_instances(self, querycontext):
        conn = self._get_bind_args(
            querycontext, self._connection_from_session, close_with_result=True
        )

        result = conn.execute(querycontext.statement, self._params)
        return loading.instances(querycontext.query, result, querycontext)

    def _execute_crud(self, stmt, mapper):
        conn = self._connection_from_session(
            mapper=mapper, clause=stmt, close_with_result=True
        )

        return conn.execute(stmt, self._params)

    def _get_bind_args(self, querycontext, fn, **kw):
        return fn(
            mapper=self._bind_mapper(), clause=querycontext.statement, **kw
        )

    @property
    def column_descriptions(self):
        """Return metadata about the columns which would be
        returned by this :class:`_query.Query`.

        Format is a list of dictionaries::

            user_alias = aliased(User, name='user2')
            q = sess.query(User, User.id, user_alias)

            # this expression:
            q.column_descriptions

            # would return:
            [
                {
                    'name':'User',
                    'type':User,
                    'aliased':False,
                    'expr':User,
                    'entity': User
                },
                {
                    'name':'id',
                    'type':Integer(),
                    'aliased':False,
                    'expr':User.id,
                    'entity': User
                },
                {
                    'name':'user2',
                    'type':User,
                    'aliased':True,
                    'expr':user_alias,
                    'entity': user_alias
                }
            ]

        """

        return [
            {
                "name": ent._label_name,
                "type": ent.type,
                "aliased": getattr(insp_ent, "is_aliased_class", False),
                "expr": ent.expr,
                "entity": getattr(insp_ent, "entity", None)
                if ent.entity_zero is not None
                and not insp_ent.is_clause_element
                else None,
            }
            for ent, insp_ent in [
                (
                    _ent,
                    (
                        inspect(_ent.entity_zero)
                        if _ent.entity_zero is not None
                        else None
                    ),
                )
                for _ent in self._entities
            ]
        ]

    def instances(self, cursor, __context=None):
        """Given a ResultProxy cursor as returned by connection.execute(),
        return an ORM result as an iterator.

        e.g.::

            result = engine.execute("select * from users")
            for u in session.query(User).instances(result):
                print u
        """
        context = __context
        if context is None:
            context = QueryContext(self)

        return loading.instances(self, cursor, context)

    def merge_result(self, iterator, load=True):
        """Merge a result into this :class:`_query.Query` object's Session.

        Given an iterator returned by a :class:`_query.Query`
        of the same structure
        as this one, return an identical iterator of results, with all mapped
        instances merged into the session using :meth:`.Session.merge`. This
        is an optimized method which will merge all mapped instances,
        preserving the structure of the result rows and unmapped columns with
        less method overhead than that of calling :meth:`.Session.merge`
        explicitly for each value.

        The structure of the results is determined based on the column list of
        this :class:`_query.Query` - if these do not correspond,
        unchecked errors
        will occur.

        The 'load' argument is the same as that of :meth:`.Session.merge`.

        For an example of how :meth:`_query.Query.merge_result` is used, see
        the source code for the example :ref:`examples_caching`, where
        :meth:`_query.Query.merge_result` is used to efficiently restore state
        from a cache back into a target :class:`.Session`.

        """

        return loading.merge_result(self, iterator, load)

    @property
    def _select_args(self):
        return {
            "limit": self._limit,
            "offset": self._offset,
            "distinct": self._distinct,
            "prefixes": self._prefixes,
            "suffixes": self._suffixes,
            "group_by": self._group_by or None,
            "having": self._having,
        }

    @property
    def _should_nest_selectable(self):
        kwargs = self._select_args
        return (
            kwargs.get("limit") is not None
            or kwargs.get("offset") is not None
            or kwargs.get("distinct", False)
            or kwargs.get("group_by", False)
        )

    def exists(self):
        """A convenience method that turns a query into an EXISTS subquery
        of the form EXISTS (SELECT 1 FROM ... WHERE ...).

        e.g.::

            q = session.query(User).filter(User.name == 'fred')
            session.query(q.exists())

        Producing SQL similar to::

            SELECT EXISTS (
                SELECT 1 FROM users WHERE users.name = :name_1
            ) AS anon_1

        The EXISTS construct is usually used in the WHERE clause::

            session.query(User.id).filter(q.exists()).scalar()

        Note that some databases such as SQL Server don't allow an
        EXISTS expression to be present in the columns clause of a
        SELECT.    To select a simple boolean value based on the exists
        as a WHERE, use :func:`.literal`::

            from sqlalchemy import literal

            session.query(literal(True)).filter(q.exists()).scalar()

        """

        # .add_columns() for the case that we are a query().select_from(X),
        # so that ".statement" can be produced (#2995) but also without
        # omitting the FROM clause from a query(X) (#2818);
        # .with_only_columns() after we have a core select() so that
        # we get just "SELECT 1" without any entities.
        return sql.exists(
            self.enable_eagerloads(False)
            .add_columns(sql.literal_column("1"))
            .with_labels()
            .statement.with_only_columns([1])
        )

    def count(self):
        r"""Return a count of rows this the SQL formed by this :class:`Query`
        would return.

        This generates the SQL for this Query as follows::

            SELECT count(1) AS count_1 FROM (
                SELECT <rest of query follows...>
            ) AS anon_1

        The above SQL returns a single row, which is the aggregate value
        of the count function; the :meth:`_query.Query.count`
        method then returns
        that single integer value.

        .. warning::

            It is important to note that the value returned by
            count() is **not the same as the number of ORM objects that this
            Query would return from a method such as the .all() method**.
            The :class:`_query.Query` object,
            when asked to return full entities,
            will **deduplicate entries based on primary key**, meaning if the
            same primary key value would appear in the results more than once,
            only one object of that primary key would be present.  This does
            not apply to a query that is against individual columns.

            .. seealso::

                :ref:`faq_query_deduplicating`

                :ref:`orm_tutorial_query_returning`

        For fine grained control over specific columns to count, to skip the
        usage of a subquery or otherwise control of the FROM clause, or to use
        other aggregate functions, use :attr:`~sqlalchemy.sql.expression.func`
        expressions in conjunction with :meth:`~.Session.query`, i.e.::

            from sqlalchemy import func

            # count User records, without
            # using a subquery.
            session.query(func.count(User.id))

            # return count of user "id" grouped
            # by "name"
            session.query(func.count(User.id)).\
                    group_by(User.name)

            from sqlalchemy import distinct

            # count distinct "name" values
            session.query(func.count(distinct(User.name)))

        """
        col = sql.func.count(sql.literal_column("*"))
        return self.from_self(col).scalar()

    def delete(self, synchronize_session="evaluate"):
        r"""Perform a bulk delete query.

        Deletes rows matched by this query from the database.

        E.g.::

            sess.query(User).filter(User.age == 25).\
                delete(synchronize_session=False)

            sess.query(User).filter(User.age == 25).\
                delete(synchronize_session='evaluate')

        .. warning:: The :meth:`_query.Query.delete`
           method is a "bulk" operation,
           which bypasses ORM unit-of-work automation in favor of greater
           performance.  **Please read all caveats and warnings below.**

        :param synchronize_session: chooses the strategy for the removal of
            matched objects from the session. Valid values are:

            ``False`` - don't synchronize the session. This option is the most
            efficient and is reliable once the session is expired, which
            typically occurs after a commit(), or explicitly using
            expire_all(). Before the expiration, objects may still remain in
            the session which were in fact deleted which can lead to confusing
            results if they are accessed via get() or already loaded
            collections.

            ``'fetch'`` - performs a select query before the delete to find
            objects that are matched by the delete query and need to be
            removed from the session. Matched objects are removed from the
            session.

            ``'evaluate'`` - Evaluate the query's criteria in Python straight
            on the objects in the session. If evaluation of the criteria isn't
            implemented, an error is raised.

            The expression evaluator currently doesn't account for differing
            string collations between the database and Python.

        :return: the count of rows matched as returned by the database's
          "row count" feature.

        .. warning:: **Additional Caveats for bulk query deletes**

            * This method does **not work for joined
              inheritance mappings**, since the **multiple table
              deletes are not supported by SQL** as well as that the
              **join condition of an inheritance mapper is not
              automatically rendered**.  Care must be taken in any
              multiple-table delete to first accommodate via some other means
              how the related table will be deleted, as well as to
              explicitly include the joining
              condition between those tables, even in mappings where
              this is normally automatic. E.g. if a class ``Engineer``
              subclasses ``Employee``, a DELETE against the ``Employee``
              table would look like::

                    session.query(Engineer).\
                        filter(Engineer.id == Employee.id).\
                        filter(Employee.name == 'dilbert').\
                        delete()

              However the above SQL will not delete from the Engineer table,
              unless an ON DELETE CASCADE rule is established in the database
              to handle it.

              Short story, **do not use this method for joined inheritance
              mappings unless you have taken the additional steps to make
              this feasible**.

            * The polymorphic identity WHERE criteria is **not** included
              for single- or
              joined- table updates - this must be added **manually** even
              for single table inheritance.

            * The method does **not** offer in-Python cascading of
              relationships - it is assumed that ON DELETE CASCADE/SET
              NULL/etc. is configured for any foreign key references
              which require it, otherwise the database may emit an
              integrity violation if foreign key references are being
              enforced.

              After the DELETE, dependent objects in the
              :class:`.Session` which were impacted by an ON DELETE
              may not contain the current state, or may have been
              deleted. This issue is resolved once the
              :class:`.Session` is expired, which normally occurs upon
              :meth:`.Session.commit` or can be forced by using
              :meth:`.Session.expire_all`.  Accessing an expired
              object whose row has been deleted will invoke a SELECT
              to locate the row; when the row is not found, an
              :class:`~sqlalchemy.orm.exc.ObjectDeletedError` is
              raised.

            * The ``'fetch'`` strategy results in an additional
              SELECT statement emitted and will significantly reduce
              performance.

            * The ``'evaluate'`` strategy performs a scan of
              all matching objects within the :class:`.Session`; if the
              contents of the :class:`.Session` are expired, such as
              via a proceeding :meth:`.Session.commit` call, **this will
              result in SELECT queries emitted for every matching object**.

            * The :meth:`.MapperEvents.before_delete` and
              :meth:`.MapperEvents.after_delete`
              events **are not invoked** from this method.  Instead, the
              :meth:`.SessionEvents.after_bulk_delete` method is provided to
              act upon a mass DELETE of entity rows.

        .. seealso::

            :meth:`_query.Query.update`

            :ref:`inserts_and_updates` - Core SQL tutorial

        """

        delete_op = persistence.BulkDelete.factory(self, synchronize_session)
        delete_op.exec_()
        return delete_op.rowcount

    def update(self, values, synchronize_session="evaluate", update_args=None):
        r"""Perform a bulk update query.

        Updates rows matched by this query in the database.

        E.g.::

            sess.query(User).filter(User.age == 25).\
                update({User.age: User.age - 10}, synchronize_session=False)

            sess.query(User).filter(User.age == 25).\
                update({"age": User.age - 10}, synchronize_session='evaluate')


        .. warning:: The :meth:`_query.Query.update`
           method is a "bulk" operation,
           which bypasses ORM unit-of-work automation in favor of greater
           performance.  **Please read all caveats and warnings below.**


        :param values: a dictionary with attributes names, or alternatively
         mapped attributes or SQL expressions, as keys, and literal
         values or sql expressions as values.   If :ref:`parameter-ordered
         mode <updates_order_parameters>` is desired, the values can be
         passed as a list of 2-tuples;
         this requires that the
         :paramref:`~sqlalchemy.sql.expression.update.preserve_parameter_order`
         flag is passed to the :paramref:`.Query.update.update_args` dictionary
         as well.

          .. versionchanged:: 1.0.0 - string names in the values dictionary
             are now resolved against the mapped entity; previously, these
             strings were passed as literal column names with no mapper-level
             translation.

        :param synchronize_session: chooses the strategy to update the
         attributes on objects in the session. Valid values are:

            ``False`` - don't synchronize the session. This option is the most
            efficient and is reliable once the session is expired, which
            typically occurs after a commit(), or explicitly using
            expire_all(). Before the expiration, updated objects may still
            remain in the session with stale values on their attributes, which
            can lead to confusing results.

            ``'fetch'`` - performs a select query before the update to find
            objects that are matched by the update query. The updated
            attributes are expired on matched objects.

            ``'evaluate'`` - Evaluate the Query's criteria in Python straight
            on the objects in the session. If evaluation of the criteria isn't
            implemented, an exception is raised.

            The expression evaluator currently doesn't account for differing
            string collations between the database and Python.

        :param update_args: Optional dictionary, if present will be passed
         to the underlying :func:`_expression.update`
         construct as the ``**kw`` for
         the object.  May be used to pass dialect-specific arguments such
         as ``mysql_limit``, as well as other special arguments such as
         :paramref:`~sqlalchemy.sql.expression.update.preserve_parameter_order`.

         .. versionadded:: 1.0.0

        :return: the count of rows matched as returned by the database's
         "row count" feature.

        .. warning:: **Additional Caveats for bulk query updates**

            * The method does **not** offer in-Python cascading of
              relationships - it is assumed that ON UPDATE CASCADE is
              configured for any foreign key references which require
              it, otherwise the database may emit an integrity
              violation if foreign key references are being enforced.

              After the UPDATE, dependent objects in the
              :class:`.Session` which were impacted by an ON UPDATE
              CASCADE may not contain the current state; this issue is
              resolved once the :class:`.Session` is expired, which
              normally occurs upon :meth:`.Session.commit` or can be
              forced by using :meth:`.Session.expire_all`.

            * The ``'fetch'`` strategy results in an additional
              SELECT statement emitted and will significantly reduce
              performance.

            * The ``'evaluate'`` strategy performs a scan of
              all matching objects within the :class:`.Session`; if the
              contents of the :class:`.Session` are expired, such as
              via a proceeding :meth:`.Session.commit` call, **this will
              result in SELECT queries emitted for every matching object**.

            * The method supports multiple table updates, as detailed
              in :ref:`multi_table_updates`, and this behavior does
              extend to support updates of joined-inheritance and
              other multiple table mappings.  However, the **join
              condition of an inheritance mapper is not
              automatically rendered**. Care must be taken in any
              multiple-table update to explicitly include the joining
              condition between those tables, even in mappings where
              this is normally automatic. E.g. if a class ``Engineer``
              subclasses ``Employee``, an UPDATE of the ``Engineer``
              local table using criteria against the ``Employee``
              local table might look like::

                    session.query(Engineer).\
                        filter(Engineer.id == Employee.id).\
                        filter(Employee.name == 'dilbert').\
                        update({"engineer_type": "programmer"})

            * The polymorphic identity WHERE criteria is **not** included
              for single- or
              joined- table updates - this must be added **manually**, even
              for single table inheritance.

            * The :meth:`.MapperEvents.before_update` and
              :meth:`.MapperEvents.after_update`
              events **are not invoked from this method**.  Instead, the
              :meth:`.SessionEvents.after_bulk_update` method is provided to
              act upon a mass UPDATE of entity rows.

        .. seealso::

            :meth:`_query.Query.delete`

            :ref:`inserts_and_updates` - Core SQL tutorial

        """

        update_args = update_args or {}
        update_op = persistence.BulkUpdate.factory(
            self, synchronize_session, values, update_args
        )
        update_op.exec_()
        return update_op.rowcount

    def _compile_context(self, labels=True):
        if self.dispatch.before_compile:
            for fn in self.dispatch.before_compile:
                new_query = fn(self)
                if new_query is not None and new_query is not self:
                    self = new_query
                    if not fn._bake_ok:
                        self._bake_ok = False

        context = QueryContext(self)

        if context.statement is not None:
            return context

        context.labels = labels

        context._for_update_arg = self._for_update_arg

        for entity in self._entities:
            entity.setup_context(self, context)

        for rec in context.create_eager_joins:
            strategy = rec[0]
            strategy(context, *rec[1:])

        if context.from_clause:
            # "load from explicit FROMs" mode,
            # i.e. when select_from() or join() is used
            context.froms = list(context.from_clause)
        # else "load from discrete FROMs" mode,
        # i.e. when each _MappedEntity has its own FROM

        if self._enable_single_crit:
            self._adjust_for_single_inheritance(context)

        if not context.primary_columns:
            if self._only_load_props:
                raise sa_exc.InvalidRequestError(
                    "No column-based properties specified for "
                    "refresh operation. Use session.expire() "
                    "to reload collections and related items."
                )
            else:
                raise sa_exc.InvalidRequestError(
                    "Query contains no columns with which to " "SELECT from."
                )

        if context.multi_row_eager_loaders and self._should_nest_selectable:
            context.statement = self._compound_eager_statement(context)
        else:
            context.statement = self._simple_statement(context)

        return context

    def _compound_eager_statement(self, context):
        # for eager joins present and LIMIT/OFFSET/DISTINCT,
        # wrap the query inside a select,
        # then append eager joins onto that

        if context.order_by:
            order_by_col_expr = sql_util.expand_column_list_from_order_by(
                context.primary_columns, context.order_by
            )
        else:
            context.order_by = None
            order_by_col_expr = []

        inner = sql.select(
            context.primary_columns + order_by_col_expr,
            context.whereclause,
            from_obj=context.froms,
            use_labels=context.labels,
            # TODO: this order_by is only needed if
            # LIMIT/OFFSET is present in self._select_args,
            # else the application on the outside is enough
            order_by=context.order_by,
            **self._select_args
        )
        # put FOR UPDATE on the inner query, where MySQL will honor it,
        # as well as if it has an OF so PostgreSQL can use it.
        inner._for_update_arg = context._for_update_arg

        for hint in self._with_hints:
            inner = inner.with_hint(*hint)

        if self._correlate:
            inner = inner.correlate(*self._correlate)

        inner = inner.alias()

        equivs = self.__all_equivs()

        context.adapter = sql_util.ColumnAdapter(inner, equivs)

        statement = sql.select(
            [inner] + context.secondary_columns, use_labels=context.labels
        )

        # Oracle however does not allow FOR UPDATE on the subquery,
        # and the Oracle dialect ignores it, plus for PostgreSQL, MySQL
        # we expect that all elements of the row are locked, so also put it
        # on the outside (except in the case of PG when OF is used)
        if (
            context._for_update_arg is not None
            and context._for_update_arg.of is None
        ):
            statement._for_update_arg = context._for_update_arg

        from_clause = inner
        for eager_join in context.eager_joins.values():
            # EagerLoader places a 'stop_on' attribute on the join,
            # giving us a marker as to where the "splice point" of
            # the join should be
            from_clause = sql_util.splice_joins(
                from_clause, eager_join, eager_join.stop_on
            )

        statement.append_from(from_clause)

        if context.order_by:
            statement.append_order_by(
                *context.adapter.copy_and_process(context.order_by)
            )

        statement.append_order_by(*context.eager_order_by)
        return statement

    def _simple_statement(self, context):
        if not context.order_by:
            context.order_by = None

        if self._distinct is True and context.order_by:
            context.primary_columns += (
                sql_util.expand_column_list_from_order_by
            )(context.primary_columns, context.order_by)
        context.froms += tuple(context.eager_joins.values())

        statement = sql.select(
            context.primary_columns + context.secondary_columns,
            context.whereclause,
            from_obj=context.froms,
            use_labels=context.labels,
            order_by=context.order_by,
            **self._select_args
        )
        statement._for_update_arg = context._for_update_arg

        for hint in self._with_hints:
            statement = statement.with_hint(*hint)

        if self._correlate:
            statement = statement.correlate(*self._correlate)

        if context.eager_order_by:
            statement.append_order_by(*context.eager_order_by)
        return statement

    def _adjust_for_single_inheritance(self, context):
        """Apply single-table-inheritance filtering.

        For all distinct single-table-inheritance mappers represented in
        the columns clause of this query, as well as the "select from entity",
        add criterion to the WHERE
        clause of the given QueryContext such that only the appropriate
        subtypes are selected from the total results.

        """

        search = set(self._mapper_adapter_map.values())
        if (
            self._select_from_entity
            and self._select_from_entity not in self._mapper_adapter_map
        ):
            insp = inspect(self._select_from_entity)
            if insp.is_aliased_class:
                adapter = insp._adapter
            else:
                adapter = None
            search = search.union([(self._select_from_entity, adapter)])

        for (ext_info, adapter) in search:
            if ext_info in self._join_entities:
                continue
            single_crit = ext_info.mapper._single_table_criterion
            if single_crit is not None:
                if adapter:
                    single_crit = adapter.traverse(single_crit)

                single_crit = self._adapt_clause(single_crit, False, False)
                context.whereclause = sql.and_(
                    sql.True_._ifnone(context.whereclause), single_crit
                )


class LockmodeArg(ForUpdateArg):
    @classmethod
    def parse_legacy_query(self, mode):
        if mode in (None, False):
            return None

        if mode == "read":
            read = True
            nowait = False
        elif mode == "update":
            read = nowait = False
        elif mode == "update_nowait":
            nowait = True
            read = False
        else:
            raise sa_exc.ArgumentError(
                "Unknown with_lockmode argument: %r" % mode
            )

        return LockmodeArg(read=read, nowait=nowait)


class _QueryEntity(object):
    """Represent an entity column returned within a Query result."""

    def __new__(cls, *args, **kwargs):
        if cls is _QueryEntity:
            entity = args[1]
            if not isinstance(entity, util.string_types) and _is_mapped_class(
                entity
            ):
                cls = _MapperEntity
            elif isinstance(entity, Bundle):
                cls = _BundleEntity
            else:
                cls = _ColumnEntity
        return object.__new__(cls)

    def _clone(self):
        q = self.__class__.__new__(self.__class__)
        q.__dict__ = self.__dict__.copy()
        return q


class _MapperEntity(_QueryEntity):
    """mapper/class/AliasedClass entity"""

    def __init__(self, query, entity):
        if not query._primary_entity:
            query._primary_entity = self
        query._entities.append(self)
        query._has_mapper_entities = True
        self.entities = [entity]
        self.expr = entity

    supports_single_entity = True

    use_id_for_hash = True

    def setup_entity(self, ext_info, aliased_adapter):
        self.mapper = ext_info.mapper
        self.aliased_adapter = aliased_adapter
        self.selectable = ext_info.selectable
        self.is_aliased_class = ext_info.is_aliased_class
        self._with_polymorphic = ext_info.with_polymorphic_mappers
        self._polymorphic_discriminator = ext_info.polymorphic_on
        self.entity_zero = ext_info
        if ext_info.is_aliased_class:
            self._label_name = self.entity_zero.name
        else:
            self._label_name = self.mapper.class_.__name__
        self.path = self.entity_zero._path_registry

    def set_with_polymorphic(
        self, query, cls_or_mappers, selectable, polymorphic_on
    ):
        """Receive an update from a call to
        :meth:`_query.Query.with_polymorphic`.

        Note the newer style of using a free standing
        ``with_polymporphic()`` construct doesn't make use of this method.


        """
        if self.is_aliased_class:
            # TODO: invalidrequest ?
            raise NotImplementedError(
                "Can't use with_polymorphic() against " "an Aliased object"
            )

        if cls_or_mappers is None:
            query._reset_polymorphic_adapter(self.mapper)
            return

        mappers, from_obj = self.mapper._with_polymorphic_args(
            cls_or_mappers, selectable
        )
        self._with_polymorphic = mappers
        self._polymorphic_discriminator = polymorphic_on

        self.selectable = from_obj
        query._mapper_loads_polymorphically_with(
            self.mapper,
            sql_util.ColumnAdapter(from_obj, self.mapper._equivalent_columns),
        )

    @property
    def type(self):
        return self.mapper.class_

    @property
    def entity_zero_or_selectable(self):
        return self.entity_zero

    def corresponds_to(self, entity):
        return _entity_corresponds_to(self.entity_zero, entity)

    def adapt_to_selectable(self, query, sel):
        query._entities.append(self)

    def _get_entity_clauses(self, query, context):

        adapter = None

        if not self.is_aliased_class:
            if query._polymorphic_adapters:
                adapter = query._polymorphic_adapters.get(self.mapper, None)
        else:
            adapter = self.aliased_adapter

        if adapter:
            if query._from_obj_alias:
                ret = adapter.wrap(query._from_obj_alias)
            else:
                ret = adapter
        else:
            ret = query._from_obj_alias

        return ret

    def row_processor(self, query, context, result):
        adapter = self._get_entity_clauses(query, context)

        if context.adapter and adapter:
            adapter = adapter.wrap(context.adapter)
        elif not adapter:
            adapter = context.adapter

        # polymorphic mappers which have concrete tables in
        # their hierarchy usually
        # require row aliasing unconditionally.
        if not adapter and self.mapper._requires_row_aliasing:
            adapter = sql_util.ColumnAdapter(
                self.selectable, self.mapper._equivalent_columns
            )

        if query._primary_entity is self:
            only_load_props = query._only_load_props
            refresh_state = context.refresh_state
        else:
            only_load_props = refresh_state = None

        _instance = loading._instance_processor(
            self.mapper,
            context,
            result,
            self.path,
            adapter,
            only_load_props=only_load_props,
            refresh_state=refresh_state,
            polymorphic_discriminator=self._polymorphic_discriminator,
        )

        return _instance, self._label_name

    def setup_context(self, query, context):
        adapter = self._get_entity_clauses(query, context)

        # if self._adapted_selectable is None:
        context.froms += (self.selectable,)

        if context.order_by is False and self.mapper.order_by:
            context.order_by = self.mapper.order_by

            # apply adaptation to the mapper's order_by if needed.
            if adapter:
                context.order_by = adapter.adapt_list(
                    util.to_list(context.order_by)
                )

        loading._setup_entity_query(
            context,
            self.mapper,
            self,
            self.path,
            adapter,
            context.primary_columns,
            with_polymorphic=self._with_polymorphic,
            only_load_props=query._only_load_props,
            polymorphic_discriminator=self._polymorphic_discriminator,
        )

    def __str__(self):
        return str(self.mapper)


@inspection._self_inspects
class Bundle(InspectionAttr):
    """A grouping of SQL expressions that are returned by a
    :class:`_query.Query` under one namespace.

    The :class:`.Bundle` essentially allows nesting of the tuple-based
    results returned by a column-oriented :class:`_query.Query` object.
    It also
    is extensible via simple subclassing, where the primary capability
    to override is that of how the set of expressions should be returned,
    allowing post-processing as well as custom return types, without
    involving ORM identity-mapped classes.

    .. versionadded:: 0.9.0

    .. seealso::

        :ref:`bundles`

    """

    single_entity = False
    """If True, queries for a single Bundle will be returned as a single
    entity, rather than an element within a keyed tuple."""

    is_clause_element = False

    is_mapper = False

    is_aliased_class = False

    def __init__(self, name, *exprs, **kw):
        r"""Construct a new :class:`.Bundle`.

        e.g.::

            bn = Bundle("mybundle", MyClass.x, MyClass.y)

            for row in session.query(bn).filter(
                    bn.c.x == 5).filter(bn.c.y == 4):
                print(row.mybundle.x, row.mybundle.y)

        :param name: name of the bundle.
        :param \*exprs: columns or SQL expressions comprising the bundle.
        :param single_entity=False: if True, rows for this :class:`.Bundle`
         can be returned as a "single entity" outside of any enclosing tuple
         in the same manner as a mapped entity.

        """
        self.name = self._label = name
        self.exprs = exprs
        self.c = self.columns = ColumnCollection()
        self.columns.update(
            (getattr(col, "key", col._label), col) for col in exprs
        )
        self.single_entity = kw.pop("single_entity", self.single_entity)

    columns = None
    """A namespace of SQL expressions referred to by this :class:`.Bundle`.

        e.g.::

            bn = Bundle("mybundle", MyClass.x, MyClass.y)

            q = sess.query(bn).filter(bn.c.x == 5)

        Nesting of bundles is also supported::

            b1 = Bundle("b1",
                    Bundle('b2', MyClass.a, MyClass.b),
                    Bundle('b3', MyClass.x, MyClass.y)
                )

            q = sess.query(b1).filter(
                b1.c.b2.c.a == 5).filter(b1.c.b3.c.y == 9)

    .. seealso::

        :attr:`.Bundle.c`

    """

    c = None
    """An alias for :attr:`.Bundle.columns`."""

    def _clone(self):
        cloned = self.__class__.__new__(self.__class__)
        cloned.__dict__.update(self.__dict__)
        return cloned

    def __clause_element__(self):
        return expression.ClauseList(group=False, *self.exprs)

    @property
    def clauses(self):
        return self.__clause_element__().clauses

    def label(self, name):
        """Provide a copy of this :class:`.Bundle` passing a new label."""

        cloned = self._clone()
        cloned.name = name
        return cloned

    def create_row_processor(self, query, procs, labels):
        """Produce the "row processing" function for this :class:`.Bundle`.

        May be overridden by subclasses.

        .. seealso::

            :ref:`bundles` - includes an example of subclassing.

        """
        keyed_tuple = util.lightweight_named_tuple("result", labels)

        def proc(row):
            return keyed_tuple([proc(row) for proc in procs])

        return proc


class _BundleEntity(_QueryEntity):
    use_id_for_hash = False

    def __init__(self, query, bundle, setup_entities=True):
        query._entities.append(self)
        self.bundle = self.expr = bundle
        self.type = type(bundle)
        self._label_name = bundle.name
        self._entities = []

        if setup_entities:
            for expr in bundle.exprs:
                if isinstance(expr, Bundle):
                    _BundleEntity(self, expr)
                else:
                    _ColumnEntity(self, expr)

        self.supports_single_entity = self.bundle.single_entity

    @property
    def mapper(self):
        ezero = self.entity_zero
        if ezero is not None:
            return ezero.mapper
        else:
            return None

    @property
    def entities(self):
        entities = []
        for ent in self._entities:
            entities.extend(ent.entities)
        return entities

    @property
    def entity_zero(self):
        for ent in self._entities:
            ezero = ent.entity_zero
            if ezero is not None:
                return ezero
        else:
            return None

    def corresponds_to(self, entity):
        # TODO: we might be able to implement this but for now
        # we are working around it
        return False

    @property
    def entity_zero_or_selectable(self):
        for ent in self._entities:
            ezero = ent.entity_zero_or_selectable
            if ezero is not None:
                return ezero
        else:
            return None

    def adapt_to_selectable(self, query, sel):
        c = _BundleEntity(query, self.bundle, setup_entities=False)
        # c._label_name = self._label_name
        # c.entity_zero = self.entity_zero
        # c.entities = self.entities

        for ent in self._entities:
            ent.adapt_to_selectable(c, sel)

    def setup_entity(self, ext_info, aliased_adapter):
        for ent in self._entities:
            ent.setup_entity(ext_info, aliased_adapter)

    def setup_context(self, query, context):
        for ent in self._entities:
            ent.setup_context(query, context)

    def row_processor(self, query, context, result):
        procs, labels = zip(
            *[
                ent.row_processor(query, context, result)
                for ent in self._entities
            ]
        )

        proc = self.bundle.create_row_processor(query, procs, labels)

        return proc, self._label_name


class _ColumnEntity(_QueryEntity):
    """Column/expression based entity."""

    def __init__(self, query, column, namespace=None):
        self.expr = column
        self.namespace = namespace
        search_entities = True
        check_column = False

        if isinstance(column, util.string_types):
            util.warn_deprecated(
                "Plain string expression passed to Query() should be "
                "explicitly declared using literal_column(); "
                "automatic coercion of this value will be removed in "
                "SQLAlchemy 1.4"
            )
            column = sql.literal_column(column)
            self._label_name = column.name
            search_entities = False
            check_column = True
            _entity = None
        elif isinstance(
            column, (attributes.QueryableAttribute, interfaces.PropComparator)
        ):
            _entity = getattr(column, "_parententity", None)
            if _entity is not None:
                search_entities = False
            self._label_name = column.key
            column = column._query_clause_element()
            check_column = True
            if isinstance(column, Bundle):
                _BundleEntity(query, column)
                return

        if not isinstance(column, sql.ColumnElement):
            if hasattr(column, "_select_iterable"):
                # break out an object like Table into
                # individual columns
                for c in column._select_iterable:
                    if c is column:
                        break
                    _ColumnEntity(query, c, namespace=column)
                else:
                    return

            raise sa_exc.InvalidRequestError(
                "SQL expression, column, or mapped entity "
                "expected - got '%r'" % (column,)
            )
        elif not check_column:
            self._label_name = getattr(column, "key", None)
            search_entities = True

        self.type = type_ = column.type
        self.use_id_for_hash = not type_.hashable

        # If the Column is unnamed, give it a
        # label() so that mutable column expressions
        # can be located in the result even
        # if the expression's identity has been changed
        # due to adaption.

        if not column._label and not getattr(column, "is_literal", False):
            column = column.label(self._label_name)

        query._entities.append(self)

        self.column = column
        self.froms = set()

        # look for ORM entities represented within the
        # given expression.  Try to count only entities
        # for columns whose FROM object is in the actual list
        # of FROMs for the overall expression - this helps
        # subqueries which were built from ORM constructs from
        # leaking out their entities into the main select construct
        self.actual_froms = list(column._from_objects)
        actual_froms = set(self.actual_froms)

        if not search_entities:
            self.entity_zero = _entity
            if _entity:
                self.entities = [_entity]
                self.mapper = _entity.mapper
            else:
                self.entities = []
                self.mapper = None
            self._from_entities = set(self.entities)
        else:
            all_elements = [
                elem
                for elem in sql_util.surface_column_elements(
                    column, include_scalar_selects=False
                )
                if "parententity" in elem._annotations
            ]

            self.entities = util.unique_list(
                [
                    elem._annotations["parententity"]
                    for elem in all_elements
                    if "parententity" in elem._annotations
                ]
            )

            self._from_entities = set(
                [
                    elem._annotations["parententity"]
                    for elem in all_elements
                    if "parententity" in elem._annotations
                    and actual_froms.intersection(elem._from_objects)
                ]
            )
            if self.entities:
                self.entity_zero = self.entities[0]
                self.mapper = self.entity_zero.mapper
            elif self.namespace is not None:
                self.entity_zero = self.namespace
                self.mapper = None
            else:
                self.entity_zero = None
                self.mapper = None

    supports_single_entity = False

    @property
    def entity_zero_or_selectable(self):
        if self.entity_zero is not None:
            return self.entity_zero
        elif self.actual_froms:
            return self.actual_froms[0]
        else:
            return None

    def adapt_to_selectable(self, query, sel):
        c = _ColumnEntity(query, sel.corresponding_column(self.column))
        c._label_name = self._label_name
        c.entity_zero = self.entity_zero
        c.entities = self.entities

    def setup_entity(self, ext_info, aliased_adapter):
        if "selectable" not in self.__dict__:
            self.selectable = ext_info.selectable

        if set(self.actual_froms).intersection(
            ext_info.selectable._from_objects
        ):
            self.froms.add(ext_info.selectable)

    def corresponds_to(self, entity):
        if self.entity_zero is None:
            return False
        elif _is_aliased_class(entity):
            # TODO: polymorphic subclasses ?
            return entity is self.entity_zero
        else:
            return not _is_aliased_class(
                self.entity_zero
            ) and entity.common_parent(self.entity_zero)

    def row_processor(self, query, context, result):
        if ("fetch_column", self) in context.attributes:
            column = context.attributes[("fetch_column", self)]
        else:
            column = query._adapt_clause(self.column, False, True)

        if column._annotations:
            # annotated columns perform more slowly in compiler and
            # result due to the __eq__() method, so use deannotated
            column = column._deannotate()

        if context.adapter:
            column = context.adapter.columns[column]

        getter = result._getter(column)
        return getter, self._label_name

    def setup_context(self, query, context):
        column = query._adapt_clause(self.column, False, True)

        if column._annotations:
            # annotated columns perform more slowly in compiler and
            # result due to the __eq__() method, so use deannotated
            column = column._deannotate()

        context.froms += tuple(self.froms)
        context.primary_columns.append(column)

        context.attributes[("fetch_column", self)] = column

    def __str__(self):
        return str(self.column)


class QueryContext(object):
    __slots__ = (
        "multi_row_eager_loaders",
        "adapter",
        "froms",
        "for_update",
        "query",
        "session",
        "autoflush",
        "populate_existing",
        "invoke_all_eagers",
        "version_check",
        "refresh_state",
        "primary_columns",
        "secondary_columns",
        "eager_order_by",
        "eager_joins",
        "create_eager_joins",
        "propagate_options",
        "attributes",
        "statement",
        "from_clause",
        "whereclause",
        "order_by",
        "labels",
        "_for_update_arg",
        "runid",
        "partials",
        "post_load_paths",
        "identity_token",
    )

    def __init__(self, query):

        if query._statement is not None:
            if (
                isinstance(query._statement, expression.SelectBase)
                and not query._statement._textual
                and not query._statement.use_labels
            ):
                self.statement = query._statement.apply_labels()
            else:
                self.statement = query._statement
        else:
            self.statement = None
            self.from_clause = query._from_obj
            self.whereclause = query._criterion
            self.order_by = query._order_by

        self.multi_row_eager_loaders = False
        self.adapter = None
        self.froms = ()
        self.for_update = None
        self.query = query
        self.session = query.session
        self.autoflush = query._autoflush
        self.populate_existing = query._populate_existing
        self.invoke_all_eagers = query._invoke_all_eagers
        self.version_check = query._version_check
        self.refresh_state = query._refresh_state
        self.primary_columns = []
        self.secondary_columns = []
        self.eager_order_by = []
        self.eager_joins = {}
        self.create_eager_joins = []
        self.propagate_options = set(
            o for o in query._with_options if o.propagate_to_loaders
        )
        self.attributes = query._attributes.copy()
        if self.refresh_state is not None:
            self.identity_token = query._refresh_identity_token
        else:
            self.identity_token = None


class AliasOption(interfaces.MapperOption):
    def __init__(self, alias):
        r"""Return a :class:`.MapperOption` that will indicate to the
        :class:`_query.Query` that the main table has been aliased.

        This is a seldom-used option to suit the
        very rare case that :func:`.contains_eager`
        is being used in conjunction with a user-defined SELECT
        statement that aliases the parent table.  E.g.::

            # define an aliased UNION called 'ulist'
            ulist = users.select(users.c.user_id==7).\
                            union(users.select(users.c.user_id>7)).\
                            alias('ulist')

            # add on an eager load of "addresses"
            statement = ulist.outerjoin(addresses).\
                            select().apply_labels()

            # create query, indicating "ulist" will be an
            # alias for the main table, "addresses"
            # property should be eager loaded
            query = session.query(User).options(
                                    contains_alias(ulist),
                                    contains_eager(User.addresses))

            # then get results via the statement
            results = query.from_statement(statement).all()

        :param alias: is the string name of an alias, or a
         :class:`_expression.Alias` object representing
         the alias.

        """
        self.alias = alias

    def process_query(self, query):
        if isinstance(self.alias, util.string_types):
            alias = query._mapper_zero().persist_selectable.alias(self.alias)
        else:
            alias = self.alias
        query._from_obj_alias = sql_util.ColumnAdapter(alias)
