text: # orm/query.py
  # Copyright (C) 2005-2015 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` class, the central
  construct used by the ORM to construct database queries.
  
  The :class:`.Query` class should not be confused with the
  :class:`.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, interfaces, object_mapper, persistence,
      exc as orm_exc, loading
  )
  from .base import _entity_descriptor, _is_aliased_class, \
      _is_mapped_class, _orm_columns, _generative
  from .path_registry import PathRegistry
  from .util import (
      AliasedClass, ORMAdapter, join as orm_join, with_parent, aliased
  )
  from .. import sql, util, log, exc as sa_exc, inspect, inspection
  from ..sql.expression import _interpret_as_from
  from ..sql import (
      util as sql_util,
      expression, visitors
  )
  from ..sql.base import ColumnCollection
  from . import properties
  
  __all__ = ['Query', 'QueryContext', 'aliased']
  
  
  _path_registry = PathRegistry.root
  
  
  @inspection._self_inspects
  @log.class_logger
  class Query(object):
      """ORM-level SQL construction object.
  
      :class:`.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` object, a copy of the former with additional
      criteria and options associated with it.
  
      :class:`.Query` objects are normally initially generated using the
      :meth:`~.Session.query` method of :class:`.Session`.  For a full
      walkthrough of :class:`.Query` usage, see the
      :ref:`ormtutorial_toplevel`.
  
      """
  
      _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
      _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
      _from_obj = ()
      _join_entities = ()
      _select_from_entity = None
      _mapper_adapter_map = {}
      _filter_aliases = None
      _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
  
      _current_path = _path_registry
  
      def __init__(self, entities, session=None):
          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
          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.mapped_table 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 = sql_util.ColumnAdapter(
                              ext_info.selectable,
                              ext_info.mapper._equivalent_columns
                          )
                      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 = from_obj
                  if set_base_alias:
                      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)
  
      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_text(o),
                  True, True)
              for o in cols
          ]
  
      @_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 ?
          orm_only = getattr(self, '_orm_only_adapt', orm_only)
  
          if as_filter and self._filter_aliases:
              for fa in self._filter_aliases._visitor_iterator:
                  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(
                  (
                      getattr(self, '_orm_only_from_obj_alias', orm_only),
                      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):
              for _orm_only, adapter in adapters:
                  # if 'orm only', look for ORM annotations
                  # in the element before adapting.
                  if not _orm_only or \
                          '_orm_adapt' in elem._annotations or \
                          "parententity" in elem._annotations:
  
                      e = adapter(elem)
                      if e is not None:
                          return e
  
          return visitors.replacement_traverse(
              clause,
              {},
              replace
          )
  
      def _entity_zero(self):
          return self._entities[0]
  
      def _mapper_zero(self):
          return self._select_from_entity or \
              self._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._mapper_zero()
          )
  
      def _mapper_zero_or_none(self):
          if self._primary_entity:
              return self._primary_entity.mapper
          else:
              return None
  
      def _only_mapper_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._mapper_zero()
  
      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 _no_select_modifiers(self, meth):
          if not self._enable_assertions:
              return
          for attr, methname, notset in (
              ('_limit', 'limit()', None),
              ('_offset', 'offset()', None),
              ('_order_by', 'order_by()', False),
              ('_group_by', 'group_by()', False),
              ('_distinct', 'distinct()', False),
          ):
              if getattr(self, attr) is not notset:
                  raise sa_exc.InvalidRequestError(
                      "Can't call Query.%s() when %s has been called" %
                      (meth, methname)
                  )
  
      def _get_options(self, populate_existing=None,
                       version_check=None,
                       only_load_props=None,
                       refresh_state=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)
          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)
  
          # TODO: there's no tests covering effects of
          # the annotation not being there
          return stmt._annotate({'no_replacement_traverse': True})
  
      def subquery(self, name=None, with_labels=False, reduce_columns=False):
          """return the full SELECT statement represented by
          this :class:`.Query`, embedded within an :class:`.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:`.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` first to apply table-qualified labels
           to all columns.
  
          :param reduce_columns: if True, :meth:`.Select.reduce_columns` will
           be called on the resulting :func:`.select` construct,
           to remove same-named columns where one also refers to the other
           via foreign key or WHERE clause equivalence.
  
           .. versionchanged:: 0.8 the ``with_labels`` and ``reduce_columns``
              keyword arguments were added.
  
          """
          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):
          """Return the full SELECT statement represented by this
          :class:`.Query` represented as a common table expression (CTE).
  
          .. versionadded:: 0.7.6
  
          Parameters and usage are the same as those of the
          :meth:`.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:`.SelectBase.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`, converted
          to a scalar subquery with a label of the given name.
  
          Analogous to :meth:`sqlalchemy.sql.expression.SelectBase.label`.
  
          .. versionadded:: 0.6.5
  
          """
  
          return self.enable_eagerloads(False).statement.label(name)
  
      def as_scalar(self):
          """Return the full SELECT statement represented by this
          :class:`.Query`, converted to a scalar subquery.
  
          Analogous to :meth:`sqlalchemy.sql.expression.SelectBase.as_scalar`.
  
          .. versionadded:: 0.6.5
  
          """
  
          return self.enable_eagerloads(False).statement.as_scalar()
  
      @property
      def selectable(self):
          """Return the :class:`.Select` object emitted by this :class:`.Query`.
  
          Used for :func:`.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 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.
  
          """
          self._enable_eagerloads = value
  
      @_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.
  
          """
          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.with_polymorphic` applies transformations
          to the "main" mapped class represented by this :class:`.Query`.
          The "main" mapped class here means the :class:`.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.
  
          .. versionchanged:: 0.8
              A new and more flexible function
              :func:`.orm.with_polymorphic` supersedes
              :meth:`.Query.with_polymorphic`, as it can apply the equivalent
              functionality to any set of columns or classes in the
              :class:`.Query`, not just the "zero mapper".  See that
              function for a description of arguments.
  
          """
  
          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):
          """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:`.yield_per` method **is not compatible with most
          eager loading schemes, including joinedload and subqueryload**.
          See the warning below.
  
          .. 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` dialect
              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.
  
          """
          self._yield_per = count
          self._execution_options = self._execution_options.union(
              {"stream_results": True})
  
      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))
  
          :meth:`~.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.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.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` must be constructed in this way,
          i.e. against a single mapped entity,
          with no additional filtering criterion.  Loading
          options via :meth:`~.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:`.relationship`, using a simple
          foreign-key-to-primary-key criterion, will also use an
          operation equivalent to :meth:`~.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 or tuple value representing
           the primary key.   For a composite primary key,
           the order of identifiers corresponds in most cases
           to that of the mapped :class:`.Table` object's
           primary key columns.  For a :func:`.mapper` that
           was given the ``primary key`` argument during
           construction, the order of identifiers corresponds
           to the elements present in this collection.
  
          :return: The object instance, or ``None``.
  
          """
  
          # convert composite types to individual args
          if hasattr(ident, '__composite_values__'):
              ident = ident.__composite_values__()
  
          ident = util.to_list(ident)
  
          mapper = self._only_full_mapper_zero("get")
  
          if len(ident) != 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))
  
          key = mapper.identity_key_from_primary_key(ident)
  
          if not self._populate_existing and \
                  not mapper.always_refresh and \
                  self._for_update_arg is None:
  
              instance = loading.get_from_identity(
                  self.session, key, attributes.PASSIVE_OFF)
              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
  
          return loading.load_on_ident(self, key)
  
      @_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:`.Select.correlate` after coercion to expression constructs.
  
          The correlation arguments take effect in such cases
          as when :meth:`.Query.from_self` is used, or when
          a subquery as returned by :meth:`.Query.subquery` is
          embedded in another :func:`~.expression.select` construct.
  
           """
  
          self._correlate = self._correlate.union(
              _interpret_as_from(s)
              if s is not None else None
              for s in args)
  
      @_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` 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.
  
          """
          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._invoke_all_eagers`.
  
          """
          self._invoke_all_eagers = value
  
      def with_parent(self, instance, property=None):
          """Add filtering criterion that relates the given instance
          to a child object or collection, using its attribute state
          as well as an established :func:`.relationship()`
          configuration.
  
          The method uses the :func:`.with_parent` function to generate
          the clause, the result of which is passed to :meth:`.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` object's target mapper.
  
          """
  
          if property is None:
              mapper = object_mapper(instance)
  
              for prop in mapper.iterate_properties:
                  if isinstance(prop, properties.RelationshipProperty) and \
                          prop.mapper is self._mapper_zero():
                      property = prop
                      break
              else:
                  raise sa_exc.InvalidRequestError(
                      "Could not locate a property which relates instances "
                      "of class '%s' to instances of class '%s'" %
                      (
                          self._mapper_zero().class_.__name__,
                          instance.__class__.__name__)
                  )
  
          return self.filter(with_parent(instance, property))
  
      @_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` that will use the given :class:`.Session`.
  
          """
  
          self.session = session
  
      def from_self(self, *entities):
          """return a Query that selects from this Query's
          SELECT statement.
  
          \*entities - optional list of entities which will replace
          those being selected.
  
          """
          fromclause = self.with_labels().enable_eagerloads(False).\
              _set_enable_single_crit(False).\
              statement.correlate(None)
          q = self._from_selectable(fromclause)
          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',
          ):
              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):
          """Return a new :class:`.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)
  
          .. versionadded:: 0.6.5
  
          """
          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 Query object, applying the given list of
          mapper options.
  
          Most supplied options regard changing how column- and
          relationship-mapped attributes are loaded. See the sections
          :ref:`deferred` and :doc:`/orm/loading_relationships` for reference
          documentation.
  
          """
          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()
          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` 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`
          objects.  See the example at :ref:`hybrid_transformers`.
  
          .. versionadded:: 0.7.4
  
          """
          return fn(self)
  
      @_generative()
      def with_hint(self, selectable, text, dialect_name='*'):
          """Add an indexing hint for the given entity or selectable to
          this :class:`.Query`.
  
          Functionality is passed straight through to
          :meth:`~sqlalchemy.sql.expression.Select.with_hint`,
          with the addition that ``selectable`` can be a
          :class:`.Table`, :class:`.Alias`, or ORM entity / mapped class
          /etc.
          """
          selectable = inspect(selectable).selectable
  
          self._with_hints += ((selectable, text, dialect_name),)
  
      @_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:`.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.
  
          """
          self._execution_options = self._execution_options.union(kwargs)
  
      @_generative()
      def with_lockmode(self, mode):
          """Return a new :class:`.Query` object with the specified "locking mode",
          which essentially refers to the ``FOR UPDATE`` clause.
  
          .. deprecated:: 0.9.0 superseded by :meth:`.Query.with_for_update`.
  
          :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.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):
          """return a new :class:`.Query` with the specified options for the
          ``FOR UPDATE`` clause.
  
          The behavior of this method is identical to that of
          :meth:`.SelectBase.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).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.with_for_update` supersedes
             the :meth:`.Query.with_lockmode` method.
  
          .. seealso::
  
              :meth:`.GenerativeSelect.with_for_update` - Core level method with
              full argument and behavioral description.
  
          """
          self._for_update_arg = LockmodeArg(read=read, nowait=nowait, of=of)
  
      @_generative()
      def params(self, *args, **kwargs):
          """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):
          """apply the given filtering criterion to a copy
          of this :class:`.Query`, using SQL expressions.
  
          e.g.::
  
              session.query(MyClass).filter(MyClass.name == 'some name')
  
          Multiple criteria are joined together by AND::
  
              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:`.text` construct.
  
          .. versionchanged:: 0.7.5
              Multiple criteria joined by AND.
  
          .. seealso::
  
              :meth:`.Query.filter_by` - filter on keyword expressions.
  
          """
          for criterion in list(criterion):
              criterion = 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):
          """apply the given filtering criterion to a copy
          of this :class:`.Query`, using keyword expressions.
  
          e.g.::
  
              session.query(MyClass).filter_by(name = 'some name')
  
          Multiple criteria are joined together by AND::
  
              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.join`.
  
          .. seealso::
  
              :meth:`.Query.filter` - filter on SQL expressions.
  
          """
  
          clauses = [_entity_descriptor(self._joinpoint_zero(), key) == value
                     for key, value in kwargs.items()]
          return self.filter(sql.and_(*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 ``Query``
  
          All existing ORDER BY settings can be suppressed by
          passing ``None`` - this will suppress any ORDER BY configured
          on mappers as well.
  
          Alternatively, an existing ORDER BY setting on the Query
          object can be entirely cancelled by passing ``False``
          as the value - use this before calling methods where
          an ORDER BY is invalid.
  
          """
  
          if len(criterion) == 1:
              if criterion[0] is False:
                  if '_order_by' in self.__dict__:
                      del self._order_by
                  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`"""
  
          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):
          """apply a HAVING criterion to the query and return the
          newly resulting :class:`.Query`.
  
          :meth:`~.Query.having` is used in conjunction with
          :meth:`~.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)
  
          """
  
          if isinstance(criterion, util.string_types):
              criterion = sql.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 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` object will not render ORDER BY within
          its SELECT statement.
  
          """
  
          return self._from_selectable(
              expression.union(*([self] + list(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._from_selectable(
              expression.union_all(*([self] + list(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._from_selectable(
              expression.intersect(*([self] + list(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._from_selectable(
              expression.intersect_all(*([self] + list(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._from_selectable(
              expression.except_(*([self] + list(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._from_selectable(
              expression.except_all(*([self] + list(q)))
          )
  
      def join(self, *props, **kwargs):
          """Create a SQL JOIN against this :class:`.Query` object's criterion
          and apply generatively, returning the newly resulting :class:`.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.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.join` along ``User.addresses``
          will result in SQL equivalent to::
  
              SELECT user.* FROM user JOIN address ON user.id = address.user_id
  
          In the above example we refer to ``User.addresses`` as passed to
          :meth:`~.Query.join` as the *on clause*, that is, it indicates
          how the "ON" portion of the JOIN should be constructed.  For a
          single-entity query such as the one above (i.e. we start by selecting
          only from ``User`` and nothing else), the relationship can also be
          specified by its string name::
  
              q = session.query(User).join("addresses")
  
          :meth:`~.Query.join` can also accommodate multiple
          "on clause" arguments to produce a chain of joins, such as below
          where a join across four related entities is constructed::
  
              q = session.query(User).join("orders", "items", "keywords")
  
          The above would be shorthand for three separate calls to
          :meth:`~.Query.join`, each using an explicit attribute to indicate
          the source entity::
  
              q = session.query(User).\\
                      join(User.orders).\\
                      join(Order.items).\\
                      join(Item.keywords)
  
          **Joins to a Target Entity or Selectable**
  
          A second form of :meth:`~.Query.join` allows any mapped entity
          or core selectable construct as a target.   In this usage,
          :meth:`~.Query.join` will attempt
          to create a JOIN along the natural foreign key relationship between
          two entities::
  
              q = session.query(User).join(Address)
  
          The above calling form of :meth:`~.Query.join` will raise an error if
          either there are no foreign keys between the two entities, or if
          there are multiple foreign key linkages between them.   In the
          above calling form, :meth:`~.Query.join` is called upon to
          create the "on clause" automatically for us.  The target can
          be any mapped entity or selectable, such as a :class:`.Table`::
  
              q = session.query(User).join(addresses_table)
  
          **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.   Suppose for
          example we wanted to join to ``Address`` twice, using
          an alias the second time.  We use :func:`~sqlalchemy.orm.aliased`
          to create a distinct alias of ``Address``, and join
          to it using the ``target, onclause`` form, so that the
          alias can be specified explicitly as the target along with
          the relationship to instruct how the ON clause should proceed::
  
              a_alias = aliased(Address)
  
              q = session.query(User).\\
                      join(User.addresses).\\
                      join(a_alias, User.addresses).\\
                      filter(Address.email_address=='ed@foo.com').\\
                      filter(a_alias.email_address=='ed@bar.com')
  
          Where above, the generated SQL would be similar to::
  
              SELECT user.* FROM user
                  JOIN address ON user.id = address.user_id
                  JOIN address AS address_1 ON user.id=address_1.user_id
                  WHERE address.email_address = :email_address_1
                  AND address_1.email_address = :email_address_2
  
          The two-argument calling form of :meth:`~.Query.join`
          also allows us to construct arbitrary joins with SQL-oriented
          "on clause" expressions, not relying upon configured relationships
          at all.  Any SQL expression can be passed as the ON clause
          when using the two-argument form, which should refer to the target
          entity in some way as well as an applicable source entity::
  
              q = session.query(User).join(Address, User.id==Address.user_id)
  
          .. versionchanged:: 0.7
              In SQLAlchemy 0.6 and earlier, the two argument form of
              :meth:`~.Query.join` requires the usage of a tuple:
              ``query(User).join((Address, User.id==Address.user_id))``\ .
              This calling form is accepted in 0.7 and further, though
              is not necessary unless multiple join conditions are passed to
              a single :meth:`~.Query.join` call, which itself is also not
              generally necessary as it is now equivalent to multiple
              calls (this wasn't always the case).
  
          **Advanced Join Targeting and Adaption**
  
          There is a lot of flexibility in what the "target" can be when using
          :meth:`~.Query.join`.   As noted previously, it also accepts
          :class:`.Table` constructs and other selectables such as
          :func:`.alias` and :func:`.select` constructs, with either the one
          or two-argument forms::
  
              addresses_q = select([Address.user_id]).\\
                          where(Address.email_address.endswith("@bar.com")).\\
                          alias()
  
              q = session.query(User).\\
                          join(addresses_q, addresses_q.c.user_id==User.id)
  
          :meth:`~.Query.join` also features the ability to *adapt* a
          :meth:`~sqlalchemy.orm.relationship` -driven ON clause to the target
          selectable. Below we construct a JOIN from ``User`` to a subquery
          against ``Address``, allowing the relationship denoted by
          ``User.addresses`` to *adapt* itself to the altered target::
  
              address_subq = session.query(Address).\\
                              filter(Address.email_address == 'ed@foo.com').\\
                              subquery()
  
              q = session.query(User).join(address_subq, User.addresses)
  
          Producing SQL similar to::
  
              SELECT user.* FROM user
                  JOIN (
                      SELECT address.id AS id,
                              address.user_id AS user_id,
                              address.email_address AS email_address
                      FROM address
                      WHERE address.email_address = :email_address_1
                  ) AS anon_1 ON user.id = anon_1.user_id
  
          The above form allows one to fall back onto an explicit ON
          clause at any time::
  
              q = session.query(User).\\
                      join(address_subq, User.id==address_subq.c.user_id)
  
          **Controlling what to Join From**
  
          While :meth:`~.Query.join` exclusively deals with the "right"
          side of the JOIN, we can also control the "left" side, in those
          cases where it's needed, using :meth:`~.Query.select_from`.
          Below we construct a query against ``Address`` but can still
          make usage of ``User.addresses`` as our ON clause by instructing
          the :class:`.Query` to select first from the ``User``
          entity::
  
              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
  
          **Constructing Aliases Anonymously**
  
          :meth:`~.Query.join` can construct anonymous aliases
          using the ``aliased=True`` flag.  This feature is useful
          when a query is being joined algorithmically, such as
          when querying self-referentially to an arbitrary depth::
  
              q = session.query(Node).\\
                      join("children", "children", aliased=True)
  
          When ``aliased=True`` is used, the actual "alias" construct
          is not explicitly available.  To work with it, methods such as
          :meth:`.Query.filter` will adapt the incoming entity to
          the last join point::
  
              q = session.query(Node).\\
                      join("children", "children", aliased=True).\\
                      filter(Node.name == 'grandchild 1')
  
          When using automatic aliasing, the ``from_joinpoint=True``
          argument can allow a multi-node join to be broken into
          multiple calls to :meth:`~.Query.join`, so that
          each path along the way can be further filtered::
  
              q = session.query(Node).\\
                      join("children", aliased=True).\\
                      filter(Node.name='child 1').\\
                      join("children", aliased=True, from_joinpoint=True).\\
                      filter(Node.name == 'grandchild 1')
  
          The filtering aliases above can then be reset back to the
          original ``Node`` entity using :meth:`~.Query.reset_joinpoint`::
  
              q = session.query(Node).\\
                      join("children", "children", aliased=True).\\
                      filter(Node.name == 'grandchild 1').\\
                      reset_joinpoint().\\
                      filter(Node.name == 'parent 1)
  
          For an example of ``aliased=True``, see the distribution
          example :ref:`examples_xmlpersistence` which illustrates
          an XPath-like query system using algorithmic joins.
  
          :param \*props: A collection of one or more join conditions,
           each consisting of a relationship-bound attribute or string
           relationship name representing an "on clause", or a single
           target entity, or a tuple in the form of ``(target, onclause)``.
           A special two-argument calling form of the form ``target, onclause``
           is also accepted.
          :param aliased=False: If True, indicate that the JOIN target should be
           anonymously aliased.  Subsequent calls to :meth:`~.Query.filter`
           and similar will adapt the incoming criterion to the target
           alias, until :meth:`~.Query.reset_joinpoint` is called.
          :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.
  
          .. seealso::
  
              :ref:`ormtutorial_joins` in the ORM tutorial.
  
              :ref:`inheritance_toplevel` for details on how
              :meth:`~.Query.join` is used for inheritance relationships.
  
              :func:`.orm.join` - a standalone ORM-level join function,
              used internally by :meth:`.Query.join`, which in previous
              SQLAlchemy versions was the primary ORM-level joining interface.
  
          """
          aliased, from_joinpoint = kwargs.pop('aliased', False),\
              kwargs.pop('from_joinpoint', False)
          if kwargs:
              raise TypeError("unknown arguments: %s" %
                              ', '.join(sorted(kwargs)))
          return self._join(props,
                            outerjoin=False, create_aliases=aliased,
                            from_joinpoint=from_joinpoint)
  
      def outerjoin(self, *props, **kwargs):
          """Create a left outer join against this ``Query`` object's criterion
          and apply generatively, returning the newly resulting ``Query``.
  
          Usage is the same as the ``join()`` method.
  
          """
          aliased, from_joinpoint = kwargs.pop('aliased', False), \
              kwargs.pop('from_joinpoint', False)
          if kwargs:
              raise TypeError("unknown arguments: %s" %
                              ', '.join(sorted(kwargs)))
          return self._join(props,
                            outerjoin=True, 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
              jp['prev'] = (f, prev)
              jp = prev
          self._joinpath = jp
  
      @_generative(_no_statement_condition, _no_limit_offset)
      def _join(self, keys, outerjoin, create_aliases, from_joinpoint):
          """consumes arguments from join() or 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,)
  
          for arg1 in util.to_list(keys):
              if isinstance(arg1, tuple):
                  # "tuple" form of join, multiple
                  # tuples are accepted as well.   The simpler
                  # "2-arg" form is preferred.  May deprecate
                  # the "tuple" usage.
                  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.  may possibly
              # lock this down to (right_entity, onclause) in 0.6.
              if isinstance(
                      arg1, (interfaces.PropComparator, util.string_types)):
                  right_entity, onclause = arg2, arg1
              else:
                  right_entity, onclause = arg1, arg2
  
              left_entity = prop = None
  
              if isinstance(onclause, interfaces.PropComparator):
                  of_type = getattr(onclause, '_of_type', None)
              else:
                  of_type = None
  
              if isinstance(onclause, util.string_types):
                  left_entity = self._joinpoint_zero()
  
                  descriptor = _entity_descriptor(left_entity, onclause)
                  onclause = descriptor
  
              # check for q.join(Class.propname, from_joinpoint=True)
              # and Class is that of the current joinpoint
              elif from_joinpoint and \
                      isinstance(onclause, interfaces.PropComparator):
                  left_entity = onclause._parententity
  
                  info = inspect(self._joinpoint_zero())
                  left_mapper, left_selectable, left_is_aliased = \
                      getattr(info, 'mapper', None), \
                      info.selectable, \
                      getattr(info, 'is_aliased_class', None)
  
                  if left_mapper is left_entity:
                      left_entity = self._joinpoint_zero()
                      descriptor = _entity_descriptor(left_entity,
                                                      onclause.key)
                      onclause = descriptor
  
              if isinstance(onclause, interfaces.PropComparator):
                  if right_entity is None:
                      if of_type:
                          right_entity = of_type
                      else:
                          right_entity = onclause.property.mapper
  
                  left_entity = onclause._parententity
  
                  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_entity, right_entity, 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)
                          continue
  
              elif onclause is not None and right_entity is None:
                  # TODO: no coverage here
                  raise NotImplementedError("query.join(a==b) not supported.")
  
              self._join_left_to_right(
                  left_entity,
                  right_entity, onclause,
                  outerjoin, create_aliases, prop)
  
      def _join_left_to_right(self, left, right,
                              onclause, outerjoin, create_aliases, prop):
          """append a JOIN to the query's from clause."""
  
          self._polymorphic_adapters = self._polymorphic_adapters.copy()
  
          if left is None:
              if self._from_obj:
                  left = self._from_obj[0]
              elif self._entities:
                  left = self._entities[0].entity_zero_or_selectable
  
          if left is None:
              raise sa_exc.InvalidRequestError(
                  "Don't know how to join from %s; please use "
                  "select_from() to establish the left "
                  "entity/selectable of this join" % self._entities[0])
  
          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))
  
          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.mapped_table, 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
              elif sql_util.selectables_overlap(l_info.selectable,
                                                r_info.selectable):
                  overlap = True
  
          if overlap and l_info.selectable is r_info.selectable:
              raise sa_exc.InvalidRequestError(
                  "Can't join table/selectable '%s' to itself" %
                  l_info.selectable)
  
          right, onclause = self._prepare_right_side(
              r_info, right, onclause,
              create_aliases,
              prop, overlap)
  
          # 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}
  
          self._join_to_left(l_info, left, right, onclause, outerjoin)
  
      def _prepare_right_side(self, r_info, right, onclause, create_aliases,
                              prop, overlap):
          info = r_info
  
          right_mapper, right_selectable, right_is_aliased = \
              getattr(info, 'mapper', None), \
              info.selectable, \
              getattr(info, 'is_aliased_class', False)
  
          if right_mapper:
              self._join_entities += (info, )
  
          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)
              )
  
          if not right_mapper and prop:
              right_mapper = prop.mapper
  
          need_adapter = False
  
          if right_mapper and right is right_selectable:
              if not right_selectable.is_derived_from(
                      right_mapper.mapped_table):
                  raise sa_exc.InvalidRequestError(
                      "Selectable '%s' is not derived from '%s'" %
                      (right_selectable.description,
                       right_mapper.mapped_table.description))
  
              if isinstance(right_selectable, expression.SelectBase):
                  # TODO: this isn't even covered now!
                  right_selectable = right_selectable.alias()
                  need_adapter = True
  
              right = aliased(right_mapper, right_selectable)
  
          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 an alias() of the right side was generated here,
          # apply an adapter to all subsequent filter() calls
          # until reset_joinpoint() is called.
          if need_adapter:
              self._filter_aliases = ORMAdapter(
                  right,
                  equivalents=right_mapper and
                  right_mapper._equivalent_columns or {},
                  chain_to=self._filter_aliases)
  
          # 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 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 aliased_entity and not create_aliases:
              self._mapper_loads_polymorphically_with(
                  right_mapper,
                  ORMAdapter(
                      right,
                      equivalents=right_mapper._equivalent_columns
                  )
              )
  
          return right, onclause
  
      def _join_to_left(self, l_info, left, right, onclause, outerjoin):
          info = l_info
          left_mapper = getattr(info, 'mapper', None)
          left_selectable = info.selectable
  
          if self._from_obj:
              replace_clause_index, clause = sql_util.find_join_source(
                  self._from_obj,
                  left_selectable)
              if clause is not None:
                  try:
                      clause = orm_join(clause,
                                        right,
                                        onclause, isouter=outerjoin)
                  except sa_exc.ArgumentError as ae:
                      raise sa_exc.InvalidRequestError(
                          "Could not find a FROM clause to join from.  "
                          "Tried joining to %s, but got: %s" % (right, ae))
  
                  self._from_obj = \
                      self._from_obj[:replace_clause_index] + \
                      (clause, ) + \
                      self._from_obj[replace_clause_index + 1:]
                  return
  
          if left_mapper:
              for ent in self._entities:
                  if ent.corresponds_to(left):
                      clause = ent.selectable
                      break
              else:
                  clause = left
          else:
              clause = left_selectable
  
          assert clause is not None
          try:
              clause = orm_join(clause, right, onclause, isouter=outerjoin)
          except sa_exc.ArgumentError as ae:
              raise sa_exc.InvalidRequestError(
                  "Could not find a FROM clause to join from.  "
                  "Tried joining to %s, but got: %s" % (right, ae))
          self._from_obj = self._from_obj + (clause,)
  
      def _reset_joinpoint(self):
          self._joinpoint = self._joinpath
          self._filter_aliases = None
  
      @_generative(_no_statement_condition)
      def reset_joinpoint(self):
          """Return a new :class:`.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.join`
          method.  See the example in :meth:`~.Query.join` for how
          this is used.
  
          """
          self._reset_joinpoint()
  
      @_generative(_no_clauseelement_condition)
      def select_from(self, *from_obj):
          """Set the FROM clause of this :class:`.Query` explicitly.
  
          :meth:`.Query.select_from` is often used in conjunction with
          :meth:`.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.join`, when no
          joinpoint is otherwise established - usually, the default "join
          point" is the leftmost entity in the :class:`~.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:`.Mapper` objects
           as well as core :class:`.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.join`
  
              :meth:`.Query.select_entity_from`
  
          """
  
          self._set_select_from(from_obj, False)
  
      @_generative(_no_clauseelement_condition)
      def select_entity_from(self, from_obj):
          """Set the FROM clause of this :class:`.Query` to a
          core selectable, applying it as a replacement FROM clause
          for corresponding mapped entities.
  
          This method is similar to the :meth:`.Query.select_from`
          method, in that it sets the FROM clause of the query.  However,
          where :meth:`.Query.select_from` only affects what is placed
          in the FROM, this method also applies the given selectable
          to replace the FROM which the selected entities would normally
          select from.
  
          The given ``from_obj`` must be an instance of a :class:`.FromClause`,
          e.g. a :func:`.select` or :class:`.Alias` construct.
  
          An example would be a :class:`.Query` that selects ``User`` entities,
          but uses :meth:`.Query.select_entity_from` to have the entities
          selected from a :func:`.select` construct instead of the
          base ``user`` table::
  
              select_stmt = select([User]).where(User.id == 7)
  
              q = session.query(User).\\
                      select_entity_from(select_stmt).\\
                      filter(User.name == 'ed')
  
          The query generated will select ``User`` entities directly
          from the given :func:`.select` construct, and will be::
  
              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
  
          Notice above that even the WHERE criterion was "adapted" such that
          the ``anon_1`` subquery effectively replaces all references to the
          ``user`` table, except for the one that it refers to internally.
  
          Compare this to :meth:`.Query.select_from`, which as of
          version 0.9, does not affect existing entities.  The
          statement below::
  
              q = session.query(User).\\
                      select_from(select_stmt).\\
                      filter(User.name == 'ed')
  
          Produces SQL where both the ``user`` table as well as the
          ``select_stmt`` construct are present as separate elements
          in the FROM clause.  No "adaptation" of the ``user`` table
          is applied::
  
              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
  
          :meth:`.Query.select_entity_from` maintains an older
          behavior of :meth:`.Query.select_from`.  In modern usage,
          similar results can also be achieved using :func:`.aliased`::
  
              select_stmt = select([User]).where(User.id == 7)
              user_from_select = aliased(User, select_stmt.alias())
  
              q = session.query(user_from_select)
  
          :param from_obj: a :class:`.FromClause` object that will replace
           the FROM clause of this :class:`.Query`.
  
          .. seealso::
  
              :meth:`.Query.select_from`
  
          .. versionadded:: 0.8
              :meth:`.Query.select_entity_from` was added to specify
              the specific behavior of entity replacement, however
              the :meth:`.Query.select_from` maintains this behavior
              as well until 0.9.
  
          """
  
          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):
          """apply LIMIT/OFFSET to the ``Query`` based on a "
          "range and return the newly resulting ``Query``."""
  
          if start is not None and stop is not None:
              self._offset = (self._offset or 0) + 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 or 0) + start
  
          if 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
  
          ``Query``.
  
          """
          self._limit = limit
  
      @_generative(_no_statement_condition)
      def offset(self, offset):
          """Apply an ``OFFSET`` to the query and return the newly resulting
          ``Query``.
  
          """
          self._offset = offset
  
      @_generative(_no_statement_condition)
      def distinct(self, *criterion):
          """Apply a ``DISTINCT`` to the query and return the newly resulting
          ``Query``.
  
          :param \*expr: optional column expressions.  When present,
           the Postgresql dialect will render a ``DISTINCT ON (<expressions>>)``
           construct.
  
          """
          if not criterion:
              self._distinct = True
          else:
              criterion = self._adapt_col_list(criterion)
              if isinstance(self._distinct, list):
                  self._distinct += criterion
              else:
                  self._distinct = criterion
  
      @_generative()
      def prefix_with(self, *prefixes):
          """Apply the prefixes to the query and return the newly resulting
          ``Query``.
  
          :param \*prefixes: optional prefixes, typically strings,
           not using any commas.   In particular is useful for MySQL keywords.
  
          e.g.::
  
              query = sess.query(User.name).\\
                  prefix_with('HIGH_PRIORITY').\\
                  prefix_with('SQL_SMALL_RESULT', 'ALL')
  
          Would render::
  
              SELECT HIGH_PRIORITY SQL_SMALL_RESULT ALL users.name AS users_name
              FROM users
  
          .. versionadded:: 0.7.7
  
          """
          if self._prefixes:
              self._prefixes += prefixes
          else:
              self._prefixes = prefixes
  
      def all(self):
          """Return the results represented by this ``Query`` as a list.
  
          This results in an execution of the underlying query.
  
          """
          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`.
  
          .. seealso::
  
              :ref:`orm_tutorial_literal_sql` - usage examples in the
              ORM tutorial
  
          """
          if isinstance(statement, util.string_types):
              statement = sql.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 ``first()`` results in an execution of the underlying query.
  
          """
          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(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 does not return object
          identities.
  
          Note that an entity query, that is, one which selects one or
          more mapped classes as opposed to individual column attributes,
          may ultimately represent many rows but only one row of
          unique entity or entities - this is a successful result for one().
  
          Calling ``one()`` results in an execution of the underlying query.
  
          .. versionchanged:: 0.6
              ``one()`` fully fetches all results instead of applying
              any kind of limit, so that the "unique"-ing of entities does not
              conceal multiple object identities.
  
          """
          ret = list(self)
  
          l = len(ret)
          if l == 1:
              return ret[0]
          elif l == 0:
              raise orm_exc.NoResultFound("No row was found for one()")
          else:
              raise orm_exc.MultipleResultsFound(
                  "Multiple rows were found for one()")
  
      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 _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._connection_from_session(
              mapper=self._mapper_zero_or_none(),
              clause=querycontext.statement,
              close_with_result=True)
  
          result = conn.execute(querycontext.statement, self._params)
          return loading.instances(self, result, querycontext)
  
      @property
      def column_descriptions(self):
          """Return metadata about the columns which would be
          returned by this :class:`.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,
                  },
                  {
                      'name':'id',
                      'type':Integer(),
                      'aliased':False,
                      'expr':User.id,
                  },
                  {
                      'name':'user2',
                      'type':User,
                      'aliased':True,
                      'expr':user_alias
                  }
              ]
  
          """
          return [
              {
                  'name': ent._label_name,
                  'type': ent.type,
                  'aliased': getattr(ent, 'is_aliased_class', False),
                  'expr': ent.expr
              }
              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` object's Session.
  
          Given an iterator returned by a :class:`.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` - 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.merge_result` is used, see
          the source code for the example :ref:`examples_caching`, where
          :meth:`~.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,
              '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))
  
      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()
  
          .. versionadded:: 0.8.1
  
          """
  
          # .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.add_columns('1').with_labels().
                            statement.with_only_columns(['1']))
  
      def count(self):
          """Return a count of rows this 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
  
          .. versionchanged:: 0.7
              The above scheme is newly refined as of 0.7b3.
  
          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'):
          """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.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**
  
              * 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.update`
  
              :ref:`inserts_and_updates` - Core SQL tutorial
  
          """
          # TODO: cascades need handling.
  
          delete_op = persistence.BulkDelete.factory(
              self, synchronize_session)
          delete_op.exec_()
          return delete_op.rowcount
  
      def update(self, values, synchronize_session='evaluate'):
          """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.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.
  
          :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.
  
          :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 :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.delete`
  
              :ref:`inserts_and_updates` - Core SQL tutorial
  
          """
  
          # TODO: value keys need to be mapped to corresponding sql cols and
          # instr.attr.s to string keys
          # TODO: updates of manytoone relationships need to be converted to
          # fk assignments
          # TODO: cascades need handling.
  
          update_op = persistence.BulkUpdate.factory(
              self, synchronize_session, values)
          update_op.exec_()
          return update_op.rowcount
  
      def _compile_context(self, labels=True):
          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(*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
              context.froms = context.froms
  
          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 = list(
                  chain(*[
                      sql_util.unwrap_order_by(o)
                      for o in 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
          )
  
          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)
  
          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 and context.order_by:
              order_by_col_expr = list(
                  chain(*[
                      sql_util.unwrap_order_by(o)
                      for o in context.order_by
                  ])
              )
              context.primary_columns += order_by_col_expr
  
          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, add criterion to the WHERE
          clause of the given QueryContext such that only the appropriate
          subtypes are selected from the total results.
  
          """
  
          for (ext_info, adapter) in set(self._mapper_adapter_map.values()):
              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)
  
      def __str__(self):
          return str(self._compile_context().statement)
  
  from ..sql.selectable import ForUpdateArg
  
  
  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)
  
          self.entities = [entity]
          self.expr = entity
  
      supports_single_entity = 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
          self.custom_rows = bool(self.mapper.dispatch.append_result)
  
      def set_with_polymorphic(self, query, cls_or_mappers,
                               selectable, polymorphic_on):
          """Receive an update from a call to 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))
  
      filter_fn = id
  
      @property
      def type(self):
          return self.mapper.class_
  
      @property
      def entity_zero_or_selectable(self):
          return self.entity_zero
  
      def corresponds_to(self, entity):
          if entity.is_aliased_class:
              if self.is_aliased_class:
                  if entity._base_alias is self.entity_zero._base_alias:
                      return True
              return False
          elif self.is_aliased_class:
              if self.entity_zero._use_mapper_path:
                  return entity in self._with_polymorphic
              else:
                  return entity is self.entity_zero
  
          return entity.common_parent(self.entity_zero)
  
      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, custom_rows):
          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:
              _instance = loading.instance_processor(
                  self.mapper,
                  context,
                  self.path,
                  adapter,
                  only_load_props=query._only_load_props,
                  refresh_state=context.refresh_state,
                  polymorphic_discriminator=self._polymorphic_discriminator
              )
          else:
              _instance = loading.instance_processor(
                  self.mapper,
                  context,
                  self.path,
                  adapter,
                  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
                      )
                  )
  
          if self._with_polymorphic:
              poly_properties = self.mapper._iterate_polymorphic_properties(
                  self._with_polymorphic)
          else:
              poly_properties = self.mapper._polymorphic_properties
  
          for value in poly_properties:
              if query._only_load_props and \
                      value.key not in query._only_load_props:
                  continue
              value.setup(
                  context,
                  self,
                  self.path,
                  adapter,
                  only_load_props=query._only_load_props,
                  column_collection=context.primary_columns
              )
  
          if self._polymorphic_discriminator is not None and \
              self._polymorphic_discriminator \
                  is not self.mapper.polymorphic_on:
  
              if adapter:
                  pd = adapter.columns[self._polymorphic_discriminator]
              else:
                  pd = self._polymorphic_discriminator
              context.primary_columns.append(pd)
  
      def __str__(self):
          return str(self.mapper)
  
  
  @inspection._self_inspects
  class Bundle(object):
      """A grouping of SQL expressions that are returned by a :class:`.Query`
      under one namespace.
  
      The :class:`.Bundle` essentially allows nesting of the tuple-based
      results returned by a column-oriented :class:`.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."""
  
      def __init__(self, name, *exprs, **kw):
          """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.c)
  
      @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.
  
          """
          def proc(row, result):
              return util.KeyedTuple(
                  [proc(row, None) for proc in procs], labels)
          return proc
  
  
  class _BundleEntity(_QueryEntity):
      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, namespace=self)
  
          self.entities = ()
  
          self.filter_fn = lambda item: item
  
          self.supports_single_entity = self.bundle.single_entity
  
      custom_rows = False
  
      @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: this seems to have no effect for
          # _ColumnEntity either
          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, custom_rows):
          procs, labels = zip(
              *[ent.row_processor(query, context, custom_rows)
                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
  
          if isinstance(column, util.string_types):
              column = sql.literal_column(column)
              self._label_name = column.name
          elif isinstance(column, (
              attributes.QueryableAttribute,
              interfaces.PropComparator
          )):
              self._label_name = column.key
              column = column._query_clause_element()
          else:
              self._label_name = getattr(column, 'key', None)
  
          if not isinstance(column, expression.ColumnElement) and \
                  hasattr(column, '_select_iterable'):
              for c in column._select_iterable:
                  if c is column:
                      break
                  _ColumnEntity(query, c, namespace=column)
              else:
                  return
          elif isinstance(column, Bundle):
              _BundleEntity(query, column)
              return
  
          if not isinstance(column, sql.ColumnElement):
              raise sa_exc.InvalidRequestError(
                  "SQL expression, column, or mapped entity "
                  "expected - got '%r'" % (column, )
              )
  
          self.type = type_ = column.type
          if type_.hashable:
              self.filter_fn = lambda item: item
          else:
              counter = util.counter()
              self.filter_fn = lambda item: counter()
  
          # 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 = actual_froms = set(column._from_objects)
  
          self.entities = util.OrderedSet(
              elem._annotations['parententity']
              for elem in visitors.iterate(column, {})
              if 'parententity' in elem._annotations
              and actual_froms.intersection(elem._from_objects)
          )
  
          if self.entities:
              self.entity_zero = list(self.entities)[0]
          elif self.namespace is not None:
              self.entity_zero = self.namespace
          else:
              self.entity_zero = None
  
      supports_single_entity = False
      custom_rows = False
  
      @property
      def entity_zero_or_selectable(self):
          if self.entity_zero is not None:
              return self.entity_zero
          elif self.actual_froms:
              return list(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
          self.froms.add(ext_info.selectable)
  
      def corresponds_to(self, entity):
          # TODO: just returning False here,
          # no tests fail
          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 _resolve_expr_against_query_aliases(self, query, expr, context):
          return query._adapt_clause(expr, False, True)
  
      def row_processor(self, query, context, custom_rows):
          column = self._resolve_expr_against_query_aliases(
              query, self.column, context)
  
          if context.adapter:
              column = context.adapter.columns[column]
  
          def proc(row, result):
              return row[column]
  
          return proc, self._label_name
  
      def setup_context(self, query, context):
          column = self._resolve_expr_against_query_aliases(
              query, self.column, context)
          context.froms += tuple(self.froms)
          context.primary_columns.append(column)
  
      def __str__(self):
          return str(self.column)
  
  
  class QueryContext(object):
      multi_row_eager_loaders = False
      adapter = None
      froms = ()
      for_update = None
  
      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.query = query
          self.session = query.session
          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()
  
  
  class AliasOption(interfaces.MapperOption):
  
      def __init__(self, alias):
          """Return a :class:`.MapperOption` that will indicate to the :class:`.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:`~.sql.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().mapped_table.alias(self.alias)
          else:
              alias = self.alias
          query._from_obj_alias = sql_util.ColumnAdapter(alias)
  