text: # orm/relationships.py
  # Copyright (C) 2005-2014 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
  
  """Heuristics related to join conditions as used in
  :func:`.relationship`.
  
  Provides the :class:`.JoinCondition` object, which encapsulates
  SQL annotation and aliasing behavior focused on the `primaryjoin`
  and `secondaryjoin` aspects of :func:`.relationship`.
  
  """
  from __future__ import absolute_import
  from .. import sql, util, exc as sa_exc, schema, log
  
  from .util import CascadeOptions, _orm_annotate, _orm_deannotate
  from . import dependency
  from . import attributes
  from ..sql.util import (
      ClauseAdapter,
      join_condition, _shallow_annotate, visit_binary_product,
      _deep_deannotate, selectables_overlap
  )
  from ..sql import operators, expression, visitors
  from .interfaces import (MANYTOMANY, MANYTOONE, ONETOMANY,
                           StrategizedProperty, PropComparator)
  from ..inspection import inspect
  from . import mapper as mapperlib
  import collections
  
  
  def remote(expr):
      """Annotate a portion of a primaryjoin expression
      with a 'remote' annotation.
  
      See the section :ref:`relationship_custom_foreign` for a
      description of use.
  
      .. versionadded:: 0.8
  
      .. seealso::
  
          :ref:`relationship_custom_foreign`
  
          :func:`.foreign`
  
      """
      return _annotate_columns(expression._clause_element_as_expr(expr),
                               {"remote": True})
  
  
  def foreign(expr):
      """Annotate a portion of a primaryjoin expression
      with a 'foreign' annotation.
  
      See the section :ref:`relationship_custom_foreign` for a
      description of use.
  
      .. versionadded:: 0.8
  
      .. seealso::
  
          :ref:`relationship_custom_foreign`
  
          :func:`.remote`
  
      """
  
      return _annotate_columns(expression._clause_element_as_expr(expr),
                               {"foreign": True})
  
  
  @log.class_logger
  @util.langhelpers.dependency_for("sqlalchemy.orm.properties")
  class RelationshipProperty(StrategizedProperty):
      """Describes an object property that holds a single item or list
      of items that correspond to a related database table.
  
      Public constructor is the :func:`.orm.relationship` function.
  
      See also:
  
      :ref:`relationship_config_toplevel`
  
      """
  
      strategy_wildcard_key = 'relationship'
  
      _dependency_processor = None
  
      def __init__(self, argument,
                   secondary=None, primaryjoin=None,
                   secondaryjoin=None,
                   foreign_keys=None,
                   uselist=None,
                   order_by=False,
                   backref=None,
                   back_populates=None,
                   post_update=False,
                   cascade=False, extension=None,
                   viewonly=False, lazy=True,
                   collection_class=None, passive_deletes=False,
                   passive_updates=True, remote_side=None,
                   enable_typechecks=True, join_depth=None,
                   comparator_factory=None,
                   single_parent=False, innerjoin=False,
                   distinct_target_key=None,
                   doc=None,
                   active_history=False,
                   cascade_backrefs=True,
                   load_on_pending=False,
                   strategy_class=None, _local_remote_pairs=None,
                   query_class=None,
                   info=None):
          """Provide a relationship between two mapped classes.
  
          This corresponds to a parent-child or associative table relationship.
          The constructed class is an instance of
          :class:`.RelationshipProperty`.
  
          A typical :func:`.relationship`, used in a classical mapping::
  
             mapper(Parent, properties={
               'children': relationship(Child)
             })
  
          Some arguments accepted by :func:`.relationship` optionally accept a
          callable function, which when called produces the desired value.
          The callable is invoked by the parent :class:`.Mapper` at "mapper
          initialization" time, which happens only when mappers are first used,
          and is assumed to be after all mappings have been constructed.  This
          can be used to resolve order-of-declaration and other dependency
          issues, such as if ``Child`` is declared below ``Parent`` in the same
          file::
  
              mapper(Parent, properties={
                  "children":relationship(lambda: Child,
                                      order_by=lambda: Child.id)
              })
  
          When using the :ref:`declarative_toplevel` extension, the Declarative
          initializer allows string arguments to be passed to
          :func:`.relationship`.  These string arguments are converted into
          callables that evaluate the string as Python code, using the
          Declarative class-registry as a namespace.  This allows the lookup of
          related classes to be automatic via their string name, and removes the
          need to import related classes at all into the local module space::
  
              from sqlalchemy.ext.declarative import declarative_base
  
              Base = declarative_base()
  
              class Parent(Base):
                  __tablename__ = 'parent'
                  id = Column(Integer, primary_key=True)
                  children = relationship("Child", order_by="Child.id")
  
          .. seealso::
  
            :ref:`relationship_config_toplevel` - Full introductory and
            reference documentation for :func:`.relationship`.
  
            :ref:`orm_tutorial_relationship` - ORM tutorial introduction.
  
          :param argument:
            a mapped class, or actual :class:`.Mapper` instance, representing
            the target of the relationship.
  
            :paramref:`~.relationship.argument` may also be passed as a callable
            function which is evaluated at mapper initialization time, and may
            be passed as a Python-evaluable string when using Declarative.
  
            .. seealso::
  
              :ref:`declarative_configuring_relationships` - further detail
              on relationship configuration when using Declarative.
  
          :param secondary:
            for a many-to-many relationship, specifies the intermediary
            table, and is typically an instance of :class:`.Table`.
            In less common circumstances, the argument may also be specified
            as an :class:`.Alias` construct, or even a :class:`.Join` construct.
  
            :paramref:`~.relationship.secondary` may
            also be passed as a callable function which is evaluated at
            mapper initialization time.  When using Declarative, it may also
            be a string argument noting the name of a :class:`.Table` that is
            present in the :class:`.MetaData` collection associated with the
            parent-mapped :class:`.Table`.
  
            The :paramref:`~.relationship.secondary` keyword argument is
            typically applied in the case where the intermediary :class:`.Table`
            is not otherwise exprssed in any direct class mapping. If the
            "secondary" table is also explicitly mapped elsewhere (e.g. as in
            :ref:`association_pattern`), one should consider applying the
            :paramref:`~.relationship.viewonly` flag so that this
            :func:`.relationship` is not used for persistence operations which
            may conflict with those of the association object pattern.
  
            .. seealso::
  
                :ref:`relationships_many_to_many` - Reference example of "many
                to many".
  
                :ref:`orm_tutorial_many_to_many` - ORM tutorial introduction to
                many-to-many relationships.
  
                :ref:`self_referential_many_to_many` - Specifics on using
                many-to-many in a self-referential case.
  
                :ref:`declarative_many_to_many` - Additional options when using
                Declarative.
  
                :ref:`association_pattern` - an alternative to
                :paramref:`~.relationship.secondary` when composing association
                table relationships, allowing additional attributes to be
                specified on the association table.
  
                :ref:`composite_secondary_join` - a lesser-used pattern which
                in some cases can enable complex :func:`.relationship` SQL
                conditions to be used.
  
            .. versionadded:: 0.9.2 :paramref:`~.relationship.secondary` works
               more effectively when referring to a :class:`.Join` instance.
  
          :param active_history=False:
            When ``True``, indicates that the "previous" value for a
            many-to-one reference should be loaded when replaced, if
            not already loaded. Normally, history tracking logic for
            simple many-to-ones only needs to be aware of the "new"
            value in order to perform a flush. This flag is available
            for applications that make use of
            :func:`.attributes.get_history` which also need to know
            the "previous" value of the attribute.
  
          :param backref:
            indicates the string name of a property to be placed on the related
            mapper's class that will handle this relationship in the other
            direction. The other property will be created automatically
            when the mappers are configured.  Can also be passed as a
            :func:`.backref` object to control the configuration of the
            new relationship.
  
            .. seealso::
  
              :ref:`relationships_backref` - Introductory documentation and
              examples.
  
              :paramref:`~.relationship.back_populates` - alternative form
              of backref specification.
  
              :func:`.backref` - allows control over :func:`.relationship`
              configuration when using :paramref:`~.relationship.backref`.
  
  
          :param back_populates:
            Takes a string name and has the same meaning as
            :paramref:`~.relationship.backref`, except the complementing
            property is **not** created automatically, and instead must be
            configured explicitly on the other mapper.  The complementing
            property should also indicate
            :paramref:`~.relationship.back_populates` to this relationship to
            ensure proper functioning.
  
            .. seealso::
  
              :ref:`relationships_backref` - Introductory documentation and
              examples.
  
              :paramref:`~.relationship.backref` - alternative form
              of backref specification.
  
          :param cascade:
            a comma-separated list of cascade rules which determines how
            Session operations should be "cascaded" from parent to child.
            This defaults to ``False``, which means the default cascade
            should be used - this default cascade is ``"save-update, merge"``.
  
            The available cascades are ``save-update``, ``merge``,
            ``expunge``, ``delete``, ``delete-orphan``, and ``refresh-expire``.
            An additional option, ``all`` indicates shorthand for
            ``"save-update, merge, refresh-expire,
            expunge, delete"``, and is often used as in ``"all, delete-orphan"``
            to indicate that related objects should follow along with the
            parent object in all cases, and be deleted when de-associated.
  
            .. seealso::
  
              :ref:`unitofwork_cascades` - Full detail on each of the available
              cascade options.
  
              :ref:`tutorial_delete_cascade` - Tutorial example describing
              a delete cascade.
  
          :param cascade_backrefs=True:
            a boolean value indicating if the ``save-update`` cascade should
            operate along an assignment event intercepted by a backref.
            When set to ``False``, the attribute managed by this relationship
            will not cascade an incoming transient object into the session of a
            persistent parent, if the event is received via backref.
  
            .. seealso::
  
              :ref:`backref_cascade` - Full discussion and examples on how
              the :paramref:`~.relationship.cascade_backrefs` option is used.
  
          :param collection_class:
            a class or callable that returns a new list-holding object. will
            be used in place of a plain list for storing elements.
  
            .. seealso::
  
              :ref:`custom_collections` - Introductory documentation and
              examples.
  
          :param comparator_factory:
            a class which extends :class:`.RelationshipProperty.Comparator`
            which provides custom SQL clause generation for comparison
            operations.
  
            .. seealso::
  
              :class:`.PropComparator` - some detail on redefining comparators
              at this level.
  
              :ref:`custom_comparators` - Brief intro to this feature.
  
  
          :param distinct_target_key=None:
            Indicate if a "subquery" eager load should apply the DISTINCT
            keyword to the innermost SELECT statement.  When left as ``None``,
            the DISTINCT keyword will be applied in those cases when the target
            columns do not comprise the full primary key of the target table.
            When set to ``True``, the DISTINCT keyword is applied to the
            innermost SELECT unconditionally.
  
            It may be desirable to set this flag to False when the DISTINCT is
            reducing performance of the innermost subquery beyond that of what
            duplicate innermost rows may be causing.
  
            .. versionadded:: 0.8.3 -
               :paramref:`~.relationship.distinct_target_key` allows the
               subquery eager loader to apply a DISTINCT modifier to the
               innermost SELECT.
  
            .. versionchanged:: 0.9.0 -
               :paramref:`~.relationship.distinct_target_key` now defaults to
               ``None``, so that the feature enables itself automatically for
               those cases where the innermost query targets a non-unique
               key.
  
            .. seealso::
  
              :ref:`loading_toplevel` - includes an introduction to subquery
              eager loading.
  
          :param doc:
            docstring which will be applied to the resulting descriptor.
  
          :param extension:
            an :class:`.AttributeExtension` instance, or list of extensions,
            which will be prepended to the list of attribute listeners for
            the resulting descriptor placed on the class.
  
            .. deprecated:: 0.7 Please see :class:`.AttributeEvents`.
  
          :param foreign_keys:
  
            a list of columns which are to be used as "foreign key"
            columns, or columns which refer to the value in a remote
            column, within the context of this :func:`.relationship`
            object's :paramref:`~.relationship.primaryjoin` condition.
            That is, if the :paramref:`~.relationship.primaryjoin`
            condition of this :func:`.relationship` is ``a.id ==
            b.a_id``, and the values in ``b.a_id`` are required to be
            present in ``a.id``, then the "foreign key" column of this
            :func:`.relationship` is ``b.a_id``.
  
            In normal cases, the :paramref:`~.relationship.foreign_keys`
            parameter is **not required.** :func:`.relationship` will
            automatically determine which columns in the
            :paramref:`~.relationship.primaryjoin` conditition are to be
            considered "foreign key" columns based on those
            :class:`.Column` objects that specify :class:`.ForeignKey`,
            or are otherwise listed as referencing columns in a
            :class:`.ForeignKeyConstraint` construct.
            :paramref:`~.relationship.foreign_keys` is only needed when:
  
              1. There is more than one way to construct a join from the local
                 table to the remote table, as there are multiple foreign key
                 references present.  Setting ``foreign_keys`` will limit the
                 :func:`.relationship` to consider just those columns specified
                 here as "foreign".
  
                 .. versionchanged:: 0.8
                      A multiple-foreign key join ambiguity can be resolved by
                      setting the :paramref:`~.relationship.foreign_keys`
                      parameter alone, without the need to explicitly set
                      :paramref:`~.relationship.primaryjoin` as well.
  
              2. The :class:`.Table` being mapped does not actually have
                 :class:`.ForeignKey` or :class:`.ForeignKeyConstraint`
                 constructs present, often because the table
                 was reflected from a database that does not support foreign key
                 reflection (MySQL MyISAM).
  
              3. The :paramref:`~.relationship.primaryjoin` argument is used to
                 construct a non-standard join condition, which makes use of
                 columns or expressions that do not normally refer to their
                 "parent" column, such as a join condition expressed by a
                 complex comparison using a SQL function.
  
            The :func:`.relationship` construct will raise informative
            error messages that suggest the use of the
            :paramref:`~.relationship.foreign_keys` parameter when
            presented with an ambiguous condition.   In typical cases,
            if :func:`.relationship` doesn't raise any exceptions, the
            :paramref:`~.relationship.foreign_keys` parameter is usually
            not needed.
  
            :paramref:`~.relationship.foreign_keys` may also be passed as a
            callable function which is evaluated at mapper initialization time,
            and may be passed as a Python-evaluable string when using
            Declarative.
  
            .. seealso::
  
              :ref:`relationship_foreign_keys`
  
              :ref:`relationship_custom_foreign`
  
              :func:`.foreign` - allows direct annotation of the "foreign"
              columns within a :paramref:`~.relationship.primaryjoin` condition.
  
            .. versionadded:: 0.8
                The :func:`.foreign` annotation can also be applied
                directly to the :paramref:`~.relationship.primaryjoin`
                expression, which is an alternate, more specific system of
                describing which columns in a particular
                :paramref:`~.relationship.primaryjoin` should be considered
                "foreign".
  
          :param info: Optional data dictionary which will be populated into the
              :attr:`.MapperProperty.info` attribute of this object.
  
              .. versionadded:: 0.8
  
          :param innerjoin=False:
            when ``True``, joined eager loads will use an inner join to join
            against related tables instead of an outer join.  The purpose
            of this option is generally one of performance, as inner joins
            generally perform better than outer joins.
  
            This flag can be set to ``True`` when the relationship references an
            object via many-to-one using local foreign keys that are not
            nullable, or when the reference is one-to-one or a collection that
            is guaranteed to have one or at least one entry.
  
            The option supports the same "nested" and "unnested" options as
            that of :paramref:`.joinedload.innerjoin`.  See that flag
            for details on nested / unnested behaviors.
  
            .. seealso::
  
              :paramref:`.joinedload.innerjoin` - the option as specified by
              loader option, including detail on nesting behavior.
  
              :ref:`what_kind_of_loading` - Discussion of some details of
              various loader options.
  
  
          :param join_depth:
            when non-``None``, an integer value indicating how many levels
            deep "eager" loaders should join on a self-referring or cyclical
            relationship.  The number counts how many times the same Mapper
            shall be present in the loading condition along a particular join
            branch.  When left at its default of ``None``, eager loaders
            will stop chaining when they encounter a the same target mapper
            which is already higher up in the chain.  This option applies
            both to joined- and subquery- eager loaders.
  
            .. seealso::
  
              :ref:`self_referential_eager_loading` - Introductory documentation
              and examples.
  
          :param lazy='select': specifies
            how the related items should be loaded.  Default value is
            ``select``.  Values include:
  
            * ``select`` - items should be loaded lazily when the property is
              first accessed, using a separate SELECT statement, or identity map
              fetch for simple many-to-one references.
  
            * ``immediate`` - items should be loaded as the parents are loaded,
              using a separate SELECT statement, or identity map fetch for
              simple many-to-one references.
  
            * ``joined`` - items should be loaded "eagerly" in the same query as
              that of the parent, using a JOIN or LEFT OUTER JOIN.  Whether
              the join is "outer" or not is determined by the
              :paramref:`~.relationship.innerjoin` parameter.
  
            * ``subquery`` - items should be loaded "eagerly" as the parents are
              loaded, using one additional SQL statement, which issues a JOIN to
              a subquery of the original statement, for each collection
              requested.
  
            * ``noload`` - no loading should occur at any time.  This is to
              support "write-only" attributes, or attributes which are
              populated in some manner specific to the application.
  
            * ``dynamic`` - the attribute will return a pre-configured
              :class:`.Query` object for all read
              operations, onto which further filtering operations can be
              applied before iterating the results.  See
              the section :ref:`dynamic_relationship` for more details.
  
            * True - a synonym for 'select'
  
            * False - a synonym for 'joined'
  
            * None - a synonym for 'noload'
  
            .. seealso::
  
              :doc:`/orm/loading` - Full documentation on relationship loader
              configuration.
  
              :ref:`dynamic_relationship` - detail on the ``dynamic`` option.
  
          :param load_on_pending=False:
            Indicates loading behavior for transient or pending parent objects.
  
            When set to ``True``, causes the lazy-loader to
            issue a query for a parent object that is not persistent, meaning it
            has never been flushed.  This may take effect for a pending object
            when autoflush is disabled, or for a transient object that has been
            "attached" to a :class:`.Session` but is not part of its pending
            collection.
  
            The :paramref:`~.relationship.load_on_pending` flag does not improve
            behavior when the ORM is used normally - object references should be
            constructed at the object level, not at the foreign key level, so
            that they are present in an ordinary way before a flush proceeds.
            This flag is not not intended for general use.
  
            .. seealso::
  
                :meth:`.Session.enable_relationship_loading` - this method
                establishes "load on pending" behavior for the whole object, and
                also allows loading on objects that remain transient or
                detached.
  
          :param order_by:
            indicates the ordering that should be applied when loading these
            items.  :paramref:`~.relationship.order_by` is expected to refer to
            one of the :class:`.Column` objects to which the target class is
            mapped, or the attribute itself bound to the target class which
            refers to the column.
  
            :paramref:`~.relationship.order_by` may also be passed as a callable
            function which is evaluated at mapper initialization time, and may
            be passed as a Python-evaluable string when using Declarative.
  
          :param passive_deletes=False:
             Indicates loading behavior during delete operations.
  
             A value of True indicates that unloaded child items should not
             be loaded during a delete operation on the parent.  Normally,
             when a parent item is deleted, all child items are loaded so
             that they can either be marked as deleted, or have their
             foreign key to the parent set to NULL.  Marking this flag as
             True usually implies an ON DELETE <CASCADE|SET NULL> rule is in
             place which will handle updating/deleting child rows on the
             database side.
  
             Additionally, setting the flag to the string value 'all' will
             disable the "nulling out" of the child foreign keys, when there
             is no delete or delete-orphan cascade enabled.  This is
             typically used when a triggering or error raise scenario is in
             place on the database side.  Note that the foreign key
             attributes on in-session child objects will not be changed
             after a flush occurs so this is a very special use-case
             setting.
  
             .. seealso::
  
                  :ref:`passive_deletes` - Introductory documentation
                  and examples.
  
          :param passive_updates=True:
            Indicates loading and INSERT/UPDATE/DELETE behavior when the
            source of a foreign key value changes (i.e. an "on update"
            cascade), which are typically the primary key columns of the
            source row.
  
            When True, it is assumed that ON UPDATE CASCADE is configured on
            the foreign key in the database, and that the database will
            handle propagation of an UPDATE from a source column to
            dependent rows.  Note that with databases which enforce
            referential integrity (i.e. PostgreSQL, MySQL with InnoDB tables),
            ON UPDATE CASCADE is required for this operation.  The
            relationship() will update the value of the attribute on related
            items which are locally present in the session during a flush.
  
            When False, it is assumed that the database does not enforce
            referential integrity and will not be issuing its own CASCADE
            operation for an update.  The relationship() will issue the
            appropriate UPDATE statements to the database in response to the
            change of a referenced key, and items locally present in the
            session during a flush will also be refreshed.
  
            This flag should probably be set to False if primary key changes
            are expected and the database in use doesn't support CASCADE
            (i.e. SQLite, MySQL MyISAM tables).
  
            .. seealso::
  
                :ref:`passive_updates` - Introductory documentation and
                examples.
  
                :paramref:`.mapper.passive_updates` - a similar flag which
                takes effect for joined-table inheritance mappings.
  
          :param post_update:
            this indicates that the relationship should be handled by a
            second UPDATE statement after an INSERT or before a
            DELETE. Currently, it also will issue an UPDATE after the
            instance was UPDATEd as well, although this technically should
            be improved. This flag is used to handle saving bi-directional
            dependencies between two individual rows (i.e. each row
            references the other), where it would otherwise be impossible to
            INSERT or DELETE both rows fully since one row exists before the
            other. Use this flag when a particular mapping arrangement will
            incur two rows that are dependent on each other, such as a table
            that has a one-to-many relationship to a set of child rows, and
            also has a column that references a single child row within that
            list (i.e. both tables contain a foreign key to each other). If
            a flush operation returns an error that a "cyclical
            dependency" was detected, this is a cue that you might want to
            use :paramref:`~.relationship.post_update` to "break" the cycle.
  
            .. seealso::
  
                :ref:`post_update` - Introductory documentation and examples.
  
          :param primaryjoin:
            a SQL expression that will be used as the primary
            join of this child object against the parent object, or in a
            many-to-many relationship the join of the primary object to the
            association table. By default, this value is computed based on the
            foreign key relationships of the parent and child tables (or
            association table).
  
            :paramref:`~.relationship.primaryjoin` may also be passed as a
            callable function which is evaluated at mapper initialization time,
            and may be passed as a Python-evaluable string when using
            Declarative.
  
            .. seealso::
  
                :ref:`relationship_primaryjoin`
  
          :param remote_side:
            used for self-referential relationships, indicates the column or
            list of columns that form the "remote side" of the relationship.
  
            :paramref:`.relationship.remote_side` may also be passed as a
            callable function which is evaluated at mapper initialization time,
            and may be passed as a Python-evaluable string when using
            Declarative.
  
            .. versionchanged:: 0.8
                The :func:`.remote` annotation can also be applied
                directly to the ``primaryjoin`` expression, which is an
                alternate, more specific system of describing which columns in a
                particular ``primaryjoin`` should be considered "remote".
  
            .. seealso::
  
              :ref:`self_referential` - in-depth explanation of how
              :paramref:`~.relationship.remote_side`
              is used to configure self-referential relationships.
  
              :func:`.remote` - an annotation function that accomplishes the
              same purpose as :paramref:`~.relationship.remote_side`, typically
              when a custom :paramref:`~.relationship.primaryjoin` condition
              is used.
  
          :param query_class:
            a :class:`.Query` subclass that will be used as the base of the
            "appender query" returned by a "dynamic" relationship, that
            is, a relationship that specifies ``lazy="dynamic"`` or was
            otherwise constructed using the :func:`.orm.dynamic_loader`
            function.
  
            .. seealso::
  
              :ref:`dynamic_relationship` - Introduction to "dynamic"
              relationship loaders.
  
          :param secondaryjoin:
            a SQL expression that will be used as the join of
            an association table to the child object. By default, this value is
            computed based on the foreign key relationships of the association
            and child tables.
  
            :paramref:`~.relationship.secondaryjoin` may also be passed as a
            callable function which is evaluated at mapper initialization time,
            and may be passed as a Python-evaluable string when using
            Declarative.
  
            .. seealso::
  
                :ref:`relationship_primaryjoin`
  
          :param single_parent:
            when True, installs a validator which will prevent objects
            from being associated with more than one parent at a time.
            This is used for many-to-one or many-to-many relationships that
            should be treated either as one-to-one or one-to-many.  Its usage
            is optional, except for :func:`.relationship` constructs which
            are many-to-one or many-to-many and also
            specify the ``delete-orphan`` cascade option.  The
            :func:`.relationship` construct itself will raise an error
            instructing when this option is required.
  
            .. seealso::
  
              :ref:`unitofwork_cascades` - includes detail on when the
              :paramref:`~.relationship.single_parent` flag may be appropriate.
  
          :param uselist:
            a boolean that indicates if this property should be loaded as a
            list or a scalar. In most cases, this value is determined
            automatically by :func:`.relationship` at mapper configuration
            time, based on the type and direction
            of the relationship - one to many forms a list, many to one
            forms a scalar, many to many is a list. If a scalar is desired
            where normally a list would be present, such as a bi-directional
            one-to-one relationship, set :paramref:`~.relationship.uselist` to
            False.
  
            The :paramref:`~.relationship.uselist` flag is also available on an
            existing :func:`.relationship` construct as a read-only attribute,
            which can be used to determine if this :func:`.relationship` deals
            with collections or scalar attributes::
  
                >>> User.addresses.property.uselist
                True
  
            .. seealso::
  
                :ref:`relationships_one_to_one` - Introduction to the "one to
                one" relationship pattern, which is typically when the
                :paramref:`~.relationship.uselist` flag is needed.
  
          :param viewonly=False:
            when set to True, the relationship is used only for loading objects,
            and not for any persistence operation.  A :func:`.relationship`
            which specifies :paramref:`~.relationship.viewonly` can work
            with a wider range of SQL operations within the
            :paramref:`~.relationship.primaryjoin` condition, including
            operations that feature the use of a variety of comparison operators
            as well as SQL functions such as :func:`~.sql.expression.cast`.  The
            :paramref:`~.relationship.viewonly` flag is also of general use when
            defining any kind of :func:`~.relationship` that doesn't represent
            the full set of related objects, to prevent modifications of the
            collection from resulting in persistence operations.
  
  
          """
  
          self.uselist = uselist
          self.argument = argument
          self.secondary = secondary
          self.primaryjoin = primaryjoin
          self.secondaryjoin = secondaryjoin
          self.post_update = post_update
          self.direction = None
          self.viewonly = viewonly
          self.lazy = lazy
          self.single_parent = single_parent
          self._user_defined_foreign_keys = foreign_keys
          self.collection_class = collection_class
          self.passive_deletes = passive_deletes
          self.cascade_backrefs = cascade_backrefs
          self.passive_updates = passive_updates
          self.remote_side = remote_side
          self.enable_typechecks = enable_typechecks
          self.query_class = query_class
          self.innerjoin = innerjoin
          self.distinct_target_key = distinct_target_key
          self.doc = doc
          self.active_history = active_history
          self.join_depth = join_depth
          self.local_remote_pairs = _local_remote_pairs
          self.extension = extension
          self.load_on_pending = load_on_pending
          self.comparator_factory = comparator_factory or \
              RelationshipProperty.Comparator
          self.comparator = self.comparator_factory(self, None)
          util.set_creation_order(self)
  
          if info is not None:
              self.info = info
  
          if strategy_class:
              self.strategy_class = strategy_class
          else:
              self.strategy_class = self._strategy_lookup(("lazy", self.lazy))
  
          self._reverse_property = set()
  
          self.cascade = cascade if cascade is not False \
              else "save-update, merge"
  
          self.order_by = order_by
  
          self.back_populates = back_populates
  
          if self.back_populates:
              if backref:
                  raise sa_exc.ArgumentError(
                      "backref and back_populates keyword arguments "
                      "are mutually exclusive")
              self.backref = None
          else:
              self.backref = backref
  
      def instrument_class(self, mapper):
          attributes.register_descriptor(
              mapper.class_,
              self.key,
              comparator=self.comparator_factory(self, mapper),
              parententity=mapper,
              doc=self.doc,
          )
  
      class Comparator(PropComparator):
          """Produce boolean, comparison, and other operators for
          :class:`.RelationshipProperty` attributes.
  
          See the documentation for :class:`.PropComparator` for a brief
          overview of ORM level operator definition.
  
          See also:
  
          :class:`.PropComparator`
  
          :class:`.ColumnProperty.Comparator`
  
          :class:`.ColumnOperators`
  
          :ref:`types_operators`
  
          :attr:`.TypeEngine.comparator_factory`
  
          """
  
          _of_type = None
  
          def __init__(
                  self, prop, parentmapper, adapt_to_entity=None, of_type=None):
              """Construction of :class:`.RelationshipProperty.Comparator`
              is internal to the ORM's attribute mechanics.
  
              """
              self.prop = prop
              self._parentmapper = parentmapper
              self._adapt_to_entity = adapt_to_entity
              if of_type:
                  self._of_type = of_type
  
          def adapt_to_entity(self, adapt_to_entity):
              return self.__class__(self.property, self._parentmapper,
                                    adapt_to_entity=adapt_to_entity,
                                    of_type=self._of_type)
  
          @util.memoized_property
          def mapper(self):
              """The target :class:`.Mapper` referred to by this
              :class:`.RelationshipProperty.Comparator`.
  
              This is the "target" or "remote" side of the
              :func:`.relationship`.
  
              """
              return self.property.mapper
  
          @util.memoized_property
          def _parententity(self):
              return self.property.parent
  
          def _source_selectable(self):
              if self._adapt_to_entity:
                  return self._adapt_to_entity.selectable
              else:
                  return self.property.parent._with_polymorphic_selectable
  
          def __clause_element__(self):
              adapt_from = self._source_selectable()
              if self._of_type:
                  of_type = inspect(self._of_type).mapper
              else:
                  of_type = None
  
              pj, sj, source, dest, \
                  secondary, target_adapter = self.property._create_joins(
                      source_selectable=adapt_from,
                      source_polymorphic=True,
                      of_type=of_type)
              if sj is not None:
                  return pj & sj
              else:
                  return pj
  
          def of_type(self, cls):
              """Produce a construct that represents a particular 'subtype' of
              attribute for the parent class.
  
              Currently this is usable in conjunction with :meth:`.Query.join`
              and :meth:`.Query.outerjoin`.
  
              """
              return RelationshipProperty.Comparator(
                  self.property,
                  self._parentmapper,
                  adapt_to_entity=self._adapt_to_entity,
                  of_type=cls)
  
          def in_(self, other):
              """Produce an IN clause - this is not implemented
              for :func:`~.orm.relationship`-based attributes at this time.
  
              """
              raise NotImplementedError('in_() not yet supported for '
                                        'relationships.  For a simple '
                                        'many-to-one, use in_() against '
                                        'the set of foreign key values.')
  
          __hash__ = None
  
          def __eq__(self, other):
              """Implement the ``==`` operator.
  
              In a many-to-one context, such as::
  
                MyClass.some_prop == <some object>
  
              this will typically produce a
              clause such as::
  
                mytable.related_id == <some id>
  
              Where ``<some id>`` is the primary key of the given
              object.
  
              The ``==`` operator provides partial functionality for non-
              many-to-one comparisons:
  
              * Comparisons against collections are not supported.
                Use :meth:`~.RelationshipProperty.Comparator.contains`.
              * Compared to a scalar one-to-many, will produce a
                clause that compares the target columns in the parent to
                the given target.
              * Compared to a scalar many-to-many, an alias
                of the association table will be rendered as
                well, forming a natural join that is part of the
                main body of the query. This will not work for
                queries that go beyond simple AND conjunctions of
                comparisons, such as those which use OR. Use
                explicit joins, outerjoins, or
                :meth:`~.RelationshipProperty.Comparator.has` for
                more comprehensive non-many-to-one scalar
                membership tests.
              * Comparisons against ``None`` given in a one-to-many
                or many-to-many context produce a NOT EXISTS clause.
  
              """
              if isinstance(other, (util.NoneType, expression.Null)):
                  if self.property.direction in [ONETOMANY, MANYTOMANY]:
                      return ~self._criterion_exists()
                  else:
                      return _orm_annotate(self.property._optimized_compare(
                          None, adapt_source=self.adapter))
              elif self.property.uselist:
                  raise sa_exc.InvalidRequestError(
                      "Can't compare a collection to an object or collection; "
                      "use contains() to test for membership.")
              else:
                  return _orm_annotate(
                      self.property._optimized_compare(
                          other, adapt_source=self.adapter))
  
          def _criterion_exists(self, criterion=None, **kwargs):
              if getattr(self, '_of_type', None):
                  info = inspect(self._of_type)
                  target_mapper, to_selectable, is_aliased_class = \
                      info.mapper, info.selectable, info.is_aliased_class
                  if self.property._is_self_referential and not \
                          is_aliased_class:
                      to_selectable = to_selectable.alias()
  
                  single_crit = target_mapper._single_table_criterion
                  if single_crit is not None:
                      if criterion is not None:
                          criterion = single_crit & criterion
                      else:
                          criterion = single_crit
              else:
                  is_aliased_class = False
                  to_selectable = None
  
              if self.adapter:
                  source_selectable = self._source_selectable()
              else:
                  source_selectable = None
  
              pj, sj, source, dest, secondary, target_adapter = \
                  self.property._create_joins(
                      dest_polymorphic=True,
                      dest_selectable=to_selectable,
                      source_selectable=source_selectable)
  
              for k in kwargs:
                  crit = getattr(self.property.mapper.class_, k) == kwargs[k]
                  if criterion is None:
                      criterion = crit
                  else:
                      criterion = criterion & crit
  
              # annotate the *local* side of the join condition, in the case
              # of pj + sj this is the full primaryjoin, in the case of just
              # pj its the local side of the primaryjoin.
              if sj is not None:
                  j = _orm_annotate(pj) & sj
              else:
                  j = _orm_annotate(pj, exclude=self.property.remote_side)
  
              if criterion is not None and target_adapter and not \
                      is_aliased_class:
                  # limit this adapter to annotated only?
                  criterion = target_adapter.traverse(criterion)
  
              # only have the "joined left side" of what we
              # return be subject to Query adaption.  The right
              # side of it is used for an exists() subquery and
              # should not correlate or otherwise reach out
              # to anything in the enclosing query.
              if criterion is not None:
                  criterion = criterion._annotate(
                      {'no_replacement_traverse': True})
  
              crit = j & sql.True_._ifnone(criterion)
  
              ex = sql.exists([1], crit, from_obj=dest).correlate_except(dest)
              if secondary is not None:
                  ex = ex.correlate_except(secondary)
              return ex
  
          def any(self, criterion=None, **kwargs):
              """Produce an expression that tests a collection against
              particular criterion, using EXISTS.
  
              An expression like::
  
                  session.query(MyClass).filter(
                      MyClass.somereference.any(SomeRelated.x==2)
                  )
  
  
              Will produce a query like::
  
                  SELECT * FROM my_table WHERE
                  EXISTS (SELECT 1 FROM related WHERE related.my_id=my_table.id
                  AND related.x=2)
  
              Because :meth:`~.RelationshipProperty.Comparator.any` uses
              a correlated subquery, its performance is not nearly as
              good when compared against large target tables as that of
              using a join.
  
              :meth:`~.RelationshipProperty.Comparator.any` is particularly
              useful for testing for empty collections::
  
                  session.query(MyClass).filter(
                      ~MyClass.somereference.any()
                  )
  
              will produce::
  
                  SELECT * FROM my_table WHERE
                  NOT EXISTS (SELECT 1 FROM related WHERE
                  related.my_id=my_table.id)
  
              :meth:`~.RelationshipProperty.Comparator.any` is only
              valid for collections, i.e. a :func:`.relationship`
              that has ``uselist=True``.  For scalar references,
              use :meth:`~.RelationshipProperty.Comparator.has`.
  
              """
              if not self.property.uselist:
                  raise sa_exc.InvalidRequestError(
                      "'any()' not implemented for scalar "
                      "attributes. Use has()."
                  )
  
              return self._criterion_exists(criterion, **kwargs)
  
          def has(self, criterion=None, **kwargs):
              """Produce an expression that tests a scalar reference against
              particular criterion, using EXISTS.
  
              An expression like::
  
                  session.query(MyClass).filter(
                      MyClass.somereference.has(SomeRelated.x==2)
                  )
  
  
              Will produce a query like::
  
                  SELECT * FROM my_table WHERE
                  EXISTS (SELECT 1 FROM related WHERE
                  related.id==my_table.related_id AND related.x=2)
  
              Because :meth:`~.RelationshipProperty.Comparator.has` uses
              a correlated subquery, its performance is not nearly as
              good when compared against large target tables as that of
              using a join.
  
              :meth:`~.RelationshipProperty.Comparator.has` is only
              valid for scalar references, i.e. a :func:`.relationship`
              that has ``uselist=False``.  For collection references,
              use :meth:`~.RelationshipProperty.Comparator.any`.
  
              """
              if self.property.uselist:
                  raise sa_exc.InvalidRequestError(
                      "'has()' not implemented for collections.  "
                      "Use any().")
              return self._criterion_exists(criterion, **kwargs)
  
          def contains(self, other, **kwargs):
              """Return a simple expression that tests a collection for
              containment of a particular item.
  
              :meth:`~.RelationshipProperty.Comparator.contains` is
              only valid for a collection, i.e. a
              :func:`~.orm.relationship` that implements
              one-to-many or many-to-many with ``uselist=True``.
  
              When used in a simple one-to-many context, an
              expression like::
  
                  MyClass.contains(other)
  
              Produces a clause like::
  
                  mytable.id == <some id>
  
              Where ``<some id>`` is the value of the foreign key
              attribute on ``other`` which refers to the primary
              key of its parent object. From this it follows that
              :meth:`~.RelationshipProperty.Comparator.contains` is
              very useful when used with simple one-to-many
              operations.
  
              For many-to-many operations, the behavior of
              :meth:`~.RelationshipProperty.Comparator.contains`
              has more caveats. The association table will be
              rendered in the statement, producing an "implicit"
              join, that is, includes multiple tables in the FROM
              clause which are equated in the WHERE clause::
  
                  query(MyClass).filter(MyClass.contains(other))
  
              Produces a query like::
  
                  SELECT * FROM my_table, my_association_table AS
                  my_association_table_1 WHERE
                  my_table.id = my_association_table_1.parent_id
                  AND my_association_table_1.child_id = <some id>
  
              Where ``<some id>`` would be the primary key of
              ``other``. From the above, it is clear that
              :meth:`~.RelationshipProperty.Comparator.contains`
              will **not** work with many-to-many collections when
              used in queries that move beyond simple AND
              conjunctions, such as multiple
              :meth:`~.RelationshipProperty.Comparator.contains`
              expressions joined by OR. In such cases subqueries or
              explicit "outer joins" will need to be used instead.
              See :meth:`~.RelationshipProperty.Comparator.any` for
              a less-performant alternative using EXISTS, or refer
              to :meth:`.Query.outerjoin` as well as :ref:`ormtutorial_joins`
              for more details on constructing outer joins.
  
              """
              if not self.property.uselist:
                  raise sa_exc.InvalidRequestError(
                      "'contains' not implemented for scalar "
                      "attributes.  Use ==")
              clause = self.property._optimized_compare(
                  other, adapt_source=self.adapter)
  
              if self.property.secondaryjoin is not None:
                  clause.negation_clause = \
                      self.__negated_contains_or_equals(other)
  
              return clause
  
          def __negated_contains_or_equals(self, other):
              if self.property.direction == MANYTOONE:
                  state = attributes.instance_state(other)
  
                  def state_bindparam(x, state, col):
                      o = state.obj()  # strong ref
                      return sql.bindparam(
                          x, unique=True, callable_=lambda:
                          self.property.mapper.
                          _get_committed_attr_by_column(o, col))
  
                  def adapt(col):
                      if self.adapter:
                          return self.adapter(col)
                      else:
                          return col
  
                  if self.property._use_get:
                      return sql.and_(*[
                          sql.or_(
                              adapt(x) != state_bindparam(adapt(x), state, y),
                              adapt(x) == None)
                          for (x, y) in self.property.local_remote_pairs])
  
              criterion = sql.and_(*[x == y for (x, y) in
                                     zip(
                  self.property.mapper.primary_key,
                  self.property.
                  mapper.
                  primary_key_from_instance(other))
              ])
              return ~self._criterion_exists(criterion)
  
          def __ne__(self, other):
              """Implement the ``!=`` operator.
  
              In a many-to-one context, such as::
  
                MyClass.some_prop != <some object>
  
              This will typically produce a clause such as::
  
                mytable.related_id != <some id>
  
              Where ``<some id>`` is the primary key of the
              given object.
  
              The ``!=`` operator provides partial functionality for non-
              many-to-one comparisons:
  
              * Comparisons against collections are not supported.
                Use
                :meth:`~.RelationshipProperty.Comparator.contains`
                in conjunction with :func:`~.expression.not_`.
              * Compared to a scalar one-to-many, will produce a
                clause that compares the target columns in the parent to
                the given target.
              * Compared to a scalar many-to-many, an alias
                of the association table will be rendered as
                well, forming a natural join that is part of the
                main body of the query. This will not work for
                queries that go beyond simple AND conjunctions of
                comparisons, such as those which use OR. Use
                explicit joins, outerjoins, or
                :meth:`~.RelationshipProperty.Comparator.has` in
                conjunction with :func:`~.expression.not_` for
                more comprehensive non-many-to-one scalar
                membership tests.
              * Comparisons against ``None`` given in a one-to-many
                or many-to-many context produce an EXISTS clause.
  
              """
              if isinstance(other, (util.NoneType, expression.Null)):
                  if self.property.direction == MANYTOONE:
                      return sql.or_(*[x != None for x in
                                       self.property._calculated_foreign_keys])
                  else:
                      return self._criterion_exists()
              elif self.property.uselist:
                  raise sa_exc.InvalidRequestError(
                      "Can't compare a collection"
                      " to an object or collection; use "
                      "contains() to test for membership.")
              else:
                  return self.__negated_contains_or_equals(other)
  
          @util.memoized_property
          def property(self):
              if mapperlib.Mapper._new_mappers:
                  mapperlib.Mapper._configure_all()
              return self.prop
  
      def _with_parent(self, instance, alias_secondary=True):
          assert instance is not None
          return self._optimized_compare(
              instance, value_is_parent=True, alias_secondary=alias_secondary)
  
      def _optimized_compare(self, value, value_is_parent=False,
                             adapt_source=None,
                             alias_secondary=True):
          if value is not None:
              value = attributes.instance_state(value)
          return self._lazy_strategy.lazy_clause(
              value,
              reverse_direction=not value_is_parent,
              alias_secondary=alias_secondary,
              adapt_source=adapt_source)
  
      def __str__(self):
          return str(self.parent.class_.__name__) + "." + self.key
  
      def merge(self,
                session,
                source_state,
                source_dict,
                dest_state,
                dest_dict,
                load, _recursive):
  
          if load:
              for r in self._reverse_property:
                  if (source_state, r) in _recursive:
                      return
  
          if "merge" not in self._cascade:
              return
  
          if self.key not in source_dict:
              return
  
          if self.uselist:
              instances = source_state.get_impl(self.key).\
                  get(source_state, source_dict)
              if hasattr(instances, '_sa_adapter'):
                  # convert collections to adapters to get a true iterator
                  instances = instances._sa_adapter
  
              if load:
                  # for a full merge, pre-load the destination collection,
                  # so that individual _merge of each item pulls from identity
                  # map for those already present.
                  # also assumes CollectionAttrbiuteImpl behavior of loading
                  # "old" list in any case
                  dest_state.get_impl(self.key).get(dest_state, dest_dict)
  
              dest_list = []
              for current in instances:
                  current_state = attributes.instance_state(current)
                  current_dict = attributes.instance_dict(current)
                  _recursive[(current_state, self)] = True
                  obj = session._merge(current_state, current_dict,
                                       load=load, _recursive=_recursive)
                  if obj is not None:
                      dest_list.append(obj)
  
              if not load:
                  coll = attributes.init_state_collection(dest_state,
                                                          dest_dict, self.key)
                  for c in dest_list:
                      coll.append_without_event(c)
              else:
                  dest_state.get_impl(self.key)._set_iterable(
                      dest_state, dest_dict, dest_list)
          else:
              current = source_dict[self.key]
              if current is not None:
                  current_state = attributes.instance_state(current)
                  current_dict = attributes.instance_dict(current)
                  _recursive[(current_state, self)] = True
                  obj = session._merge(current_state, current_dict,
                                       load=load, _recursive=_recursive)
              else:
                  obj = None
  
              if not load:
                  dest_dict[self.key] = obj
              else:
                  dest_state.get_impl(self.key).set(dest_state,
                                                    dest_dict, obj, None)
  
      def _value_as_iterable(self, state, dict_, key,
                             passive=attributes.PASSIVE_OFF):
          """Return a list of tuples (state, obj) for the given
          key.
  
          returns an empty list if the value is None/empty/PASSIVE_NO_RESULT
          """
  
          impl = state.manager[key].impl
          x = impl.get(state, dict_, passive=passive)
          if x is attributes.PASSIVE_NO_RESULT or x is None:
              return []
          elif hasattr(impl, 'get_collection'):
              return [
                  (attributes.instance_state(o), o) for o in
                  impl.get_collection(state, dict_, x, passive=passive)
              ]
          else:
              return [(attributes.instance_state(x), x)]
  
      def cascade_iterator(self, type_, state, dict_,
                           visited_states, halt_on=None):
          # assert type_ in self._cascade
  
          # only actively lazy load on the 'delete' cascade
          if type_ != 'delete' or self.passive_deletes:
              passive = attributes.PASSIVE_NO_INITIALIZE
          else:
              passive = attributes.PASSIVE_OFF
  
          if type_ == 'save-update':
              tuples = state.manager[self.key].impl.\
                  get_all_pending(state, dict_)
  
          else:
              tuples = self._value_as_iterable(state, dict_, self.key,
                                               passive=passive)
  
          skip_pending = type_ == 'refresh-expire' and 'delete-orphan' \
              not in self._cascade
  
          for instance_state, c in tuples:
              if instance_state in visited_states:
                  continue
  
              if c is None:
                  # would like to emit a warning here, but
                  # would not be consistent with collection.append(None)
                  # current behavior of silently skipping.
                  # see [ticket:2229]
                  continue
  
              instance_dict = attributes.instance_dict(c)
  
              if halt_on and halt_on(instance_state):
                  continue
  
              if skip_pending and not instance_state.key:
                  continue
  
              instance_mapper = instance_state.manager.mapper
  
              if not instance_mapper.isa(self.mapper.class_manager.mapper):
                  raise AssertionError("Attribute '%s' on class '%s' "
                                       "doesn't handle objects "
                                       "of type '%s'" % (
                                           self.key,
                                           self.parent.class_,
                                           c.__class__
                                       ))
  
              visited_states.add(instance_state)
  
              yield c, instance_mapper, instance_state, instance_dict
  
      def _add_reverse_property(self, key):
          other = self.mapper.get_property(key, _configure_mappers=False)
          self._reverse_property.add(other)
          other._reverse_property.add(self)
  
          if not other.mapper.common_parent(self.parent):
              raise sa_exc.ArgumentError(
                  'reverse_property %r on '
                  'relationship %s references relationship %s, which '
                  'does not reference mapper %s' %
                  (key, self, other, self.parent))
  
          if self.direction in (ONETOMANY, MANYTOONE) and self.direction \
                  == other.direction:
              raise sa_exc.ArgumentError(
                  '%s and back-reference %s are '
                  'both of the same direction %r.  Did you mean to '
                  'set remote_side on the many-to-one side ?' %
                  (other, self, self.direction))
  
      @util.memoized_property
      def mapper(self):
          """Return the targeted :class:`.Mapper` for this
          :class:`.RelationshipProperty`.
  
          This is a lazy-initializing static attribute.
  
          """
          if util.callable(self.argument) and \
                  not isinstance(self.argument, (type, mapperlib.Mapper)):
              argument = self.argument()
          else:
              argument = self.argument
  
          if isinstance(argument, type):
              mapper_ = mapperlib.class_mapper(argument,
                                               configure=False)
          elif isinstance(self.argument, mapperlib.Mapper):
              mapper_ = argument
          else:
              raise sa_exc.ArgumentError(
                  "relationship '%s' expects "
                  "a class or a mapper argument (received: %s)"
                  % (self.key, type(argument)))
          return mapper_
  
      @util.memoized_property
      @util.deprecated("0.7", "Use .target")
      def table(self):
          """Return the selectable linked to this
          :class:`.RelationshipProperty` object's target
          :class:`.Mapper`.
          """
          return self.target
  
      def do_init(self):
          self._check_conflicts()
          self._process_dependent_arguments()
          self._setup_join_conditions()
          self._check_cascade_settings(self._cascade)
          self._post_init()
          self._generate_backref()
          super(RelationshipProperty, self).do_init()
          self._lazy_strategy = self._get_strategy((("lazy", "select"),))
  
      def _process_dependent_arguments(self):
          """Convert incoming configuration arguments to their
          proper form.
  
          Callables are resolved, ORM annotations removed.
  
          """
          # accept callables for other attributes which may require
          # deferred initialization.  This technique is used
          # by declarative "string configs" and some recipes.
          for attr in (
              'order_by', 'primaryjoin', 'secondaryjoin',
              'secondary', '_user_defined_foreign_keys', 'remote_side',
          ):
              attr_value = getattr(self, attr)
              if util.callable(attr_value):
                  setattr(self, attr, attr_value())
  
          # remove "annotations" which are present if mapped class
          # descriptors are used to create the join expression.
          for attr in 'primaryjoin', 'secondaryjoin':
              val = getattr(self, attr)
              if val is not None:
                  setattr(self, attr, _orm_deannotate(
                      expression._only_column_elements(val, attr))
                  )
  
          # ensure expressions in self.order_by, foreign_keys,
          # remote_side are all columns, not strings.
          if self.order_by is not False and self.order_by is not None:
              self.order_by = [
                  expression._only_column_elements(x, "order_by")
                  for x in
                  util.to_list(self.order_by)]
  
          self._user_defined_foreign_keys = \
              util.column_set(
                  expression._only_column_elements(x, "foreign_keys")
                  for x in util.to_column_set(
                      self._user_defined_foreign_keys
                  ))
  
          self.remote_side = \
              util.column_set(
                  expression._only_column_elements(x, "remote_side")
                  for x in
                  util.to_column_set(self.remote_side))
  
          self.target = self.mapper.mapped_table
  
      def _setup_join_conditions(self):
          self._join_condition = jc = JoinCondition(
              parent_selectable=self.parent.mapped_table,
              child_selectable=self.mapper.mapped_table,
              parent_local_selectable=self.parent.local_table,
              child_local_selectable=self.mapper.local_table,
              primaryjoin=self.primaryjoin,
              secondary=self.secondary,
              secondaryjoin=self.secondaryjoin,
              parent_equivalents=self.parent._equivalent_columns,
              child_equivalents=self.mapper._equivalent_columns,
              consider_as_foreign_keys=self._user_defined_foreign_keys,
              local_remote_pairs=self.local_remote_pairs,
              remote_side=self.remote_side,
              self_referential=self._is_self_referential,
              prop=self,
              support_sync=not self.viewonly,
              can_be_synced_fn=self._columns_are_mapped
          )
          self.primaryjoin = jc.deannotated_primaryjoin
          self.secondaryjoin = jc.deannotated_secondaryjoin
          self.direction = jc.direction
          self.local_remote_pairs = jc.local_remote_pairs
          self.remote_side = jc.remote_columns
          self.local_columns = jc.local_columns
          self.synchronize_pairs = jc.synchronize_pairs
          self._calculated_foreign_keys = jc.foreign_key_columns
          self.secondary_synchronize_pairs = jc.secondary_synchronize_pairs
  
      def _check_conflicts(self):
          """Test that this relationship is legal, warn about
          inheritance conflicts."""
  
          if self.parent.non_primary and not mapperlib.class_mapper(
                  self.parent.class_,
                  configure=False).has_property(self.key):
              raise sa_exc.ArgumentError(
                  "Attempting to assign a new "
                  "relationship '%s' to a non-primary mapper on "
                  "class '%s'.  New relationships can only be added "
                  "to the primary mapper, i.e. the very first mapper "
                  "created for class '%s' " %
                  (self.key, self.parent.class_.__name__,
                   self.parent.class_.__name__))
  
          # check for conflicting relationship() on superclass
          if not self.parent.concrete:
              for inheriting in self.parent.iterate_to_root():
                  if inheriting is not self.parent \
                          and inheriting.has_property(self.key):
                      util.warn("Warning: relationship '%s' on mapper "
                                "'%s' supersedes the same relationship "
                                "on inherited mapper '%s'; this can "
                                "cause dependency issues during flush"
                                % (self.key, self.parent, inheriting))
  
      def _get_cascade(self):
          """Return the current cascade setting for this
          :class:`.RelationshipProperty`.
          """
          return self._cascade
  
      def _set_cascade(self, cascade):
          cascade = CascadeOptions(cascade)
          if 'mapper' in self.__dict__:
              self._check_cascade_settings(cascade)
          self._cascade = cascade
  
          if self._dependency_processor:
              self._dependency_processor.cascade = cascade
  
      cascade = property(_get_cascade, _set_cascade)
  
      def _check_cascade_settings(self, cascade):
          if cascade.delete_orphan and not self.single_parent \
              and (self.direction is MANYTOMANY or self.direction
                   is MANYTOONE):
              raise sa_exc.ArgumentError(
                  'On %s, delete-orphan cascade is not supported '
                  'on a many-to-many or many-to-one relationship '
                  'when single_parent is not set.   Set '
                  'single_parent=True on the relationship().'
                  % self)
          if self.direction is MANYTOONE and self.passive_deletes:
              util.warn("On %s, 'passive_deletes' is normally configured "
                        "on one-to-many, one-to-one, many-to-many "
                        "relationships only."
                        % self)
  
          if self.passive_deletes == 'all' and \
              ("delete" in cascade or
               "delete-orphan" in cascade):
              raise sa_exc.ArgumentError(
                  "On %s, can't set passive_deletes='all' in conjunction "
                  "with 'delete' or 'delete-orphan' cascade" % self)
  
          if cascade.delete_orphan:
              self.mapper.primary_mapper()._delete_orphans.append(
                  (self.key, self.parent.class_)
              )
  
      def _columns_are_mapped(self, *cols):
          """Return True if all columns in the given collection are
          mapped by the tables referenced by this :class:`.Relationship`.
  
          """
          for c in cols:
              if self.secondary is not None \
                      and self.secondary.c.contains_column(c):
                  continue
              if not self.parent.mapped_table.c.contains_column(c) and \
                      not self.target.c.contains_column(c):
                  return False
          return True
  
      def _generate_backref(self):
          """Interpret the 'backref' instruction to create a
          :func:`.relationship` complementary to this one."""
  
          if self.parent.non_primary:
              return
          if self.backref is not None and not self.back_populates:
              if isinstance(self.backref, util.string_types):
                  backref_key, kwargs = self.backref, {}
              else:
                  backref_key, kwargs = self.backref
              mapper = self.mapper.primary_mapper()
  
              check = set(mapper.iterate_to_root()).\
                  union(mapper.self_and_descendants)
              for m in check:
                  if m.has_property(backref_key):
                      raise sa_exc.ArgumentError(
                          "Error creating backref "
                          "'%s' on relationship '%s': property of that "
                          "name exists on mapper '%s'" %
                          (backref_key, self, m))
  
              # determine primaryjoin/secondaryjoin for the
              # backref.  Use the one we had, so that
              # a custom join doesn't have to be specified in
              # both directions.
              if self.secondary is not None:
                  # for many to many, just switch primaryjoin/
                  # secondaryjoin.   use the annotated
                  # pj/sj on the _join_condition.
                  pj = kwargs.pop(
                      'primaryjoin',
                      self._join_condition.secondaryjoin_minus_local)
                  sj = kwargs.pop(
                      'secondaryjoin',
                      self._join_condition.primaryjoin_minus_local)
              else:
                  pj = kwargs.pop(
                      'primaryjoin',
                      self._join_condition.primaryjoin_reverse_remote)
                  sj = kwargs.pop('secondaryjoin', None)
                  if sj:
                      raise sa_exc.InvalidRequestError(
                          "Can't assign 'secondaryjoin' on a backref "
                          "against a non-secondary relationship."
                      )
  
              foreign_keys = kwargs.pop('foreign_keys',
                                        self._user_defined_foreign_keys)
              parent = self.parent.primary_mapper()
              kwargs.setdefault('viewonly', self.viewonly)
              kwargs.setdefault('post_update', self.post_update)
              kwargs.setdefault('passive_updates', self.passive_updates)
              self.back_populates = backref_key
              relationship = RelationshipProperty(
                  parent, self.secondary,
                  pj, sj,
                  foreign_keys=foreign_keys,
                  back_populates=self.key,
                  **kwargs)
              mapper._configure_property(backref_key, relationship)
  
          if self.back_populates:
              self._add_reverse_property(self.back_populates)
  
      def _post_init(self):
          if self.uselist is None:
              self.uselist = self.direction is not MANYTOONE
          if not self.viewonly:
              self._dependency_processor = \
                  dependency.DependencyProcessor.from_relationship(self)
  
      @util.memoized_property
      def _use_get(self):
          """memoize the 'use_get' attribute of this RelationshipLoader's
          lazyloader."""
  
          strategy = self._lazy_strategy
          return strategy.use_get
  
      @util.memoized_property
      def _is_self_referential(self):
          return self.mapper.common_parent(self.parent)
  
      def _create_joins(self, source_polymorphic=False,
                        source_selectable=None, dest_polymorphic=False,
                        dest_selectable=None, of_type=None):
          if source_selectable is None:
              if source_polymorphic and self.parent.with_polymorphic:
                  source_selectable = self.parent._with_polymorphic_selectable
  
          aliased = False
          if dest_selectable is None:
              if dest_polymorphic and self.mapper.with_polymorphic:
                  dest_selectable = self.mapper._with_polymorphic_selectable
                  aliased = True
              else:
                  dest_selectable = self.mapper.mapped_table
  
              if self._is_self_referential and source_selectable is None:
                  dest_selectable = dest_selectable.alias()
                  aliased = True
          else:
              aliased = True
  
          dest_mapper = of_type or self.mapper
  
          single_crit = dest_mapper._single_table_criterion
          aliased = aliased or (source_selectable is not None)
  
          primaryjoin, secondaryjoin, secondary, target_adapter, dest_selectable = \
              self._join_condition.join_targets(
                  source_selectable, dest_selectable, aliased, single_crit
              )
          if source_selectable is None:
              source_selectable = self.parent.local_table
          if dest_selectable is None:
              dest_selectable = self.mapper.local_table
          return (primaryjoin, secondaryjoin, source_selectable,
                  dest_selectable, secondary, target_adapter)
  
  
  def _annotate_columns(element, annotations):
      def clone(elem):
          if isinstance(elem, expression.ColumnClause):
              elem = elem._annotate(annotations.copy())
          elem._copy_internals(clone=clone)
          return elem
  
      if element is not None:
          element = clone(element)
      return element
  
  
  class JoinCondition(object):
      def __init__(self,
                   parent_selectable,
                   child_selectable,
                   parent_local_selectable,
                   child_local_selectable,
                   primaryjoin=None,
                   secondary=None,
                   secondaryjoin=None,
                   parent_equivalents=None,
                   child_equivalents=None,
                   consider_as_foreign_keys=None,
                   local_remote_pairs=None,
                   remote_side=None,
                   self_referential=False,
                   prop=None,
                   support_sync=True,
                   can_be_synced_fn=lambda *c: True
                   ):
          self.parent_selectable = parent_selectable
          self.parent_local_selectable = parent_local_selectable
          self.child_selectable = child_selectable
          self.child_local_selectable = child_local_selectable
          self.parent_equivalents = parent_equivalents
          self.child_equivalents = child_equivalents
          self.primaryjoin = primaryjoin
          self.secondaryjoin = secondaryjoin
          self.secondary = secondary
          self.consider_as_foreign_keys = consider_as_foreign_keys
          self._local_remote_pairs = local_remote_pairs
          self._remote_side = remote_side
          self.prop = prop
          self.self_referential = self_referential
          self.support_sync = support_sync
          self.can_be_synced_fn = can_be_synced_fn
          self._determine_joins()
          self._annotate_fks()
          self._annotate_remote()
          self._annotate_local()
          self._setup_pairs()
          self._check_foreign_cols(self.primaryjoin, True)
          if self.secondaryjoin is not None:
              self._check_foreign_cols(self.secondaryjoin, False)
          self._determine_direction()
          self._check_remote_side()
          self._log_joins()
  
      def _log_joins(self):
          if self.prop is None:
              return
          log = self.prop.logger
          log.info('%s setup primary join %s', self.prop,
                   self.primaryjoin)
          log.info('%s setup secondary join %s', self.prop,
                   self.secondaryjoin)
          log.info('%s synchronize pairs [%s]', self.prop,
                   ','.join('(%s => %s)' % (l, r) for (l, r) in
                            self.synchronize_pairs))
          log.info('%s secondary synchronize pairs [%s]', self.prop,
                   ','.join('(%s => %s)' % (l, r) for (l, r) in
                            self.secondary_synchronize_pairs or []))
          log.info('%s local/remote pairs [%s]', self.prop,
                   ','.join('(%s / %s)' % (l, r) for (l, r) in
                            self.local_remote_pairs))
          log.info('%s remote columns [%s]', self.prop,
                   ','.join('%s' % col for col in self.remote_columns)
                   )
          log.info('%s local columns [%s]', self.prop,
                   ','.join('%s' % col for col in self.local_columns)
                   )
          log.info('%s relationship direction %s', self.prop,
                   self.direction)
  
      def _determine_joins(self):
          """Determine the 'primaryjoin' and 'secondaryjoin' attributes,
          if not passed to the constructor already.
  
          This is based on analysis of the foreign key relationships
          between the parent and target mapped selectables.
  
          """
          if self.secondaryjoin is not None and self.secondary is None:
              raise sa_exc.ArgumentError(
                  "Property %s specified with secondary "
                  "join condition but "
                  "no secondary argument" % self.prop)
  
          # find a join between the given mapper's mapped table and
          # the given table. will try the mapper's local table first
          # for more specificity, then if not found will try the more
          # general mapped table, which in the case of inheritance is
          # a join.
          try:
              consider_as_foreign_keys = self.consider_as_foreign_keys or None
              if self.secondary is not None:
                  if self.secondaryjoin is None:
                      self.secondaryjoin = \
                          join_condition(
                              self.child_selectable,
                              self.secondary,
                              a_subset=self.child_local_selectable,
                              consider_as_foreign_keys=consider_as_foreign_keys
                          )
                  if self.primaryjoin is None:
                      self.primaryjoin = \
                          join_condition(
                              self.parent_selectable,
                              self.secondary,
                              a_subset=self.parent_local_selectable,
                              consider_as_foreign_keys=consider_as_foreign_keys
                          )
              else:
                  if self.primaryjoin is None:
                      self.primaryjoin = \
                          join_condition(
                              self.parent_selectable,
                              self.child_selectable,
                              a_subset=self.parent_local_selectable,
                              consider_as_foreign_keys=consider_as_foreign_keys
                          )
          except sa_exc.NoForeignKeysError:
              if self.secondary is not None:
                  raise sa_exc.NoForeignKeysError(
                      "Could not determine join "
                      "condition between parent/child tables on "
                      "relationship %s - there are no foreign keys "
                      "linking these tables via secondary table '%s'.  "
                      "Ensure that referencing columns are associated "
                      "with a ForeignKey or ForeignKeyConstraint, or "
                      "specify 'primaryjoin' and 'secondaryjoin' "
                      "expressions." % (self.prop, self.secondary))
              else:
                  raise sa_exc.NoForeignKeysError(
                      "Could not determine join "
                      "condition between parent/child tables on "
                      "relationship %s - there are no foreign keys "
                      "linking these tables.  "
                      "Ensure that referencing columns are associated "
                      "with a ForeignKey or ForeignKeyConstraint, or "
                      "specify a 'primaryjoin' expression." % self.prop)
          except sa_exc.AmbiguousForeignKeysError:
              if self.secondary is not None:
                  raise sa_exc.AmbiguousForeignKeysError(
                      "Could not determine join "
                      "condition between parent/child tables on "
                      "relationship %s - there are multiple foreign key "
                      "paths linking the tables via secondary table '%s'.  "
                      "Specify the 'foreign_keys' "
                      "argument, providing a list of those columns which "
                      "should be counted as containing a foreign key "
                      "reference from the secondary table to each of the "
                      "parent and child tables."
                      % (self.prop, self.secondary))
              else:
                  raise sa_exc.AmbiguousForeignKeysError(
                      "Could not determine join "
                      "condition between parent/child tables on "
                      "relationship %s - there are multiple foreign key "
                      "paths linking the tables.  Specify the "
                      "'foreign_keys' argument, providing a list of those "
                      "columns which should be counted as containing a "
                      "foreign key reference to the parent table."
                      % self.prop)
  
      @property
      def primaryjoin_minus_local(self):
          return _deep_deannotate(self.primaryjoin, values=("local", "remote"))
  
      @property
      def secondaryjoin_minus_local(self):
          return _deep_deannotate(self.secondaryjoin,
                                  values=("local", "remote"))
  
      @util.memoized_property
      def primaryjoin_reverse_remote(self):
          """Return the primaryjoin condition suitable for the
          "reverse" direction.
  
          If the primaryjoin was delivered here with pre-existing
          "remote" annotations, the local/remote annotations
          are reversed.  Otherwise, the local/remote annotations
          are removed.
  
          """
          if self._has_remote_annotations:
              def replace(element):
                  if "remote" in element._annotations:
                      v = element._annotations.copy()
                      del v['remote']
                      v['local'] = True
                      return element._with_annotations(v)
                  elif "local" in element._annotations:
                      v = element._annotations.copy()
                      del v['local']
                      v['remote'] = True
                      return element._with_annotations(v)
              return visitors.replacement_traverse(
                  self.primaryjoin, {}, replace)
          else:
              if self._has_foreign_annotations:
                  # TODO: coverage
                  return _deep_deannotate(self.primaryjoin,
                                          values=("local", "remote"))
              else:
                  return _deep_deannotate(self.primaryjoin)
  
      def _has_annotation(self, clause, annotation):
          for col in visitors.iterate(clause, {}):
              if annotation in col._annotations:
                  return True
          else:
              return False
  
      @util.memoized_property
      def _has_foreign_annotations(self):
          return self._has_annotation(self.primaryjoin, "foreign")
  
      @util.memoized_property
      def _has_remote_annotations(self):
          return self._has_annotation(self.primaryjoin, "remote")
  
      def _annotate_fks(self):
          """Annotate the primaryjoin and secondaryjoin
          structures with 'foreign' annotations marking columns
          considered as foreign.
  
          """
          if self._has_foreign_annotations:
              return
  
          if self.consider_as_foreign_keys:
              self._annotate_from_fk_list()
          else:
              self._annotate_present_fks()
  
      def _annotate_from_fk_list(self):
          def check_fk(col):
              if col in self.consider_as_foreign_keys:
                  return col._annotate({"foreign": True})
          self.primaryjoin = visitors.replacement_traverse(
              self.primaryjoin,
              {},
              check_fk
          )
          if self.secondaryjoin is not None:
              self.secondaryjoin = visitors.replacement_traverse(
                  self.secondaryjoin,
                  {},
                  check_fk
              )
  
      def _annotate_present_fks(self):
          if self.secondary is not None:
              secondarycols = util.column_set(self.secondary.c)
          else:
              secondarycols = set()
  
          def is_foreign(a, b):
              if isinstance(a, schema.Column) and \
                      isinstance(b, schema.Column):
                  if a.references(b):
                      return a
                  elif b.references(a):
                      return b
  
              if secondarycols:
                  if a in secondarycols and b not in secondarycols:
                      return a
                  elif b in secondarycols and a not in secondarycols:
                      return b
  
          def visit_binary(binary):
              if not isinstance(binary.left, sql.ColumnElement) or \
                      not isinstance(binary.right, sql.ColumnElement):
                  return
  
              if "foreign" not in binary.left._annotations and \
                      "foreign" not in binary.right._annotations:
                  col = is_foreign(binary.left, binary.right)
                  if col is not None:
                      if col.compare(binary.left):
                          binary.left = binary.left._annotate(
                              {"foreign": True})
                      elif col.compare(binary.right):
                          binary.right = binary.right._annotate(
                              {"foreign": True})
  
          self.primaryjoin = visitors.cloned_traverse(
              self.primaryjoin,
              {},
              {"binary": visit_binary}
          )
          if self.secondaryjoin is not None:
              self.secondaryjoin = visitors.cloned_traverse(
                  self.secondaryjoin,
                  {},
                  {"binary": visit_binary}
              )
  
      def _refers_to_parent_table(self):
          """Return True if the join condition contains column
          comparisons where both columns are in both tables.
  
          """
          pt = self.parent_selectable
          mt = self.child_selectable
          result = [False]
  
          def visit_binary(binary):
              c, f = binary.left, binary.right
              if (
                  isinstance(c, expression.ColumnClause) and
                  isinstance(f, expression.ColumnClause) and
                  pt.is_derived_from(c.table) and
                  pt.is_derived_from(f.table) and
                  mt.is_derived_from(c.table) and
                  mt.is_derived_from(f.table)
              ):
                  result[0] = True
          visitors.traverse(
              self.primaryjoin,
              {},
              {"binary": visit_binary}
          )
          return result[0]
  
      def _tables_overlap(self):
          """Return True if parent/child tables have some overlap."""
  
          return selectables_overlap(
              self.parent_selectable, self.child_selectable)
  
      def _annotate_remote(self):
          """Annotate the primaryjoin and secondaryjoin
          structures with 'remote' annotations marking columns
          considered as part of the 'remote' side.
  
          """
          if self._has_remote_annotations:
              return
  
          if self.secondary is not None:
              self._annotate_remote_secondary()
          elif self._local_remote_pairs or self._remote_side:
              self._annotate_remote_from_args()
          elif self._refers_to_parent_table():
              self._annotate_selfref(lambda col: "foreign" in col._annotations, False)
          elif self._tables_overlap():
              self._annotate_remote_with_overlap()
          else:
              self._annotate_remote_distinct_selectables()
  
      def _annotate_remote_secondary(self):
          """annotate 'remote' in primaryjoin, secondaryjoin
          when 'secondary' is present.
  
          """
          def repl(element):
              if self.secondary.c.contains_column(element):
                  return element._annotate({"remote": True})
          self.primaryjoin = visitors.replacement_traverse(
              self.primaryjoin, {}, repl)
          self.secondaryjoin = visitors.replacement_traverse(
              self.secondaryjoin, {}, repl)
  
      def _annotate_selfref(self, fn, remote_side_given):
          """annotate 'remote' in primaryjoin, secondaryjoin
          when the relationship is detected as self-referential.
  
          """
          def visit_binary(binary):
              equated = binary.left.compare(binary.right)
              if isinstance(binary.left, expression.ColumnClause) and \
                      isinstance(binary.right, expression.ColumnClause):
                  # assume one to many - FKs are "remote"
                  if fn(binary.left):
                      binary.left = binary.left._annotate({"remote": True})
                  if fn(binary.right) and not equated:
                      binary.right = binary.right._annotate(
                          {"remote": True})
              elif not remote_side_given:
                  self._warn_non_column_elements()
  
          self.primaryjoin = visitors.cloned_traverse(
              self.primaryjoin, {},
              {"binary": visit_binary})
  
      def _annotate_remote_from_args(self):
          """annotate 'remote' in primaryjoin, secondaryjoin
          when the 'remote_side' or '_local_remote_pairs'
          arguments are used.
  
          """
          if self._local_remote_pairs:
              if self._remote_side:
                  raise sa_exc.ArgumentError(
                      "remote_side argument is redundant "
                      "against more detailed _local_remote_side "
                      "argument.")
  
              remote_side = [r for (l, r) in self._local_remote_pairs]
          else:
              remote_side = self._remote_side
  
          if self._refers_to_parent_table():
              self._annotate_selfref(lambda col: col in remote_side, True)
          else:
              def repl(element):
                  if element in remote_side:
                      return element._annotate({"remote": True})
              self.primaryjoin = visitors.replacement_traverse(
                  self.primaryjoin, {}, repl)
  
      def _annotate_remote_with_overlap(self):
          """annotate 'remote' in primaryjoin, secondaryjoin
          when the parent/child tables have some set of
          tables in common, though is not a fully self-referential
          relationship.
  
          """
          def visit_binary(binary):
              binary.left, binary.right = proc_left_right(binary.left,
                                                          binary.right)
              binary.right, binary.left = proc_left_right(binary.right,
                                                          binary.left)
  
          def proc_left_right(left, right):
              if isinstance(left, expression.ColumnClause) and \
                      isinstance(right, expression.ColumnClause):
                  if self.child_selectable.c.contains_column(right) and \
                          self.parent_selectable.c.contains_column(left):
                      right = right._annotate({"remote": True})
              else:
                  self._warn_non_column_elements()
  
              return left, right
  
          self.primaryjoin = visitors.cloned_traverse(
              self.primaryjoin, {},
              {"binary": visit_binary})
  
      def _annotate_remote_distinct_selectables(self):
          """annotate 'remote' in primaryjoin, secondaryjoin
          when the parent/child tables are entirely
          separate.
  
          """
          def repl(element):
              if self.child_selectable.c.contains_column(element) and \
                      (not self.parent_local_selectable.c.
                          contains_column(element) or
                          self.child_local_selectable.c.
                          contains_column(element)):
                  return element._annotate({"remote": True})
          self.primaryjoin = visitors.replacement_traverse(
              self.primaryjoin, {}, repl)
  
      def _warn_non_column_elements(self):
          util.warn(
              "Non-simple column elements in primary "
              "join condition for property %s - consider using "
              "remote() annotations to mark the remote side."
              % self.prop
          )
  
      def _annotate_local(self):
          """Annotate the primaryjoin and secondaryjoin
          structures with 'local' annotations.
  
          This annotates all column elements found
          simultaneously in the parent table
          and the join condition that don't have a
          'remote' annotation set up from
          _annotate_remote() or user-defined.
  
          """
          if self._has_annotation(self.primaryjoin, "local"):
              return
  
          if self._local_remote_pairs:
              local_side = util.column_set([l for (l, r)
                                            in self._local_remote_pairs])
          else:
              local_side = util.column_set(self.parent_selectable.c)
  
          def locals_(elem):
              if "remote" not in elem._annotations and \
                      elem in local_side:
                  return elem._annotate({"local": True})
          self.primaryjoin = visitors.replacement_traverse(
              self.primaryjoin, {}, locals_
          )
  
      def _check_remote_side(self):
          if not self.local_remote_pairs:
              raise sa_exc.ArgumentError(
                  'Relationship %s could '
                  'not determine any unambiguous local/remote column '
                  'pairs based on join condition and remote_side '
                  'arguments.  '
                  'Consider using the remote() annotation to '
                  'accurately mark those elements of the join '
                  'condition that are on the remote side of '
                  'the relationship.' % (self.prop, ))
  
      def _check_foreign_cols(self, join_condition, primary):
          """Check the foreign key columns collected and emit error
          messages."""
  
          can_sync = False
  
          foreign_cols = self._gather_columns_with_annotation(
              join_condition, "foreign")
  
          has_foreign = bool(foreign_cols)
  
          if primary:
              can_sync = bool(self.synchronize_pairs)
          else:
              can_sync = bool(self.secondary_synchronize_pairs)
  
          if self.support_sync and can_sync or \
                  (not self.support_sync and has_foreign):
              return
  
          # from here below is just determining the best error message
          # to report.  Check for a join condition using any operator
          # (not just ==), perhaps they need to turn on "viewonly=True".
          if self.support_sync and has_foreign and not can_sync:
              err = "Could not locate any simple equality expressions "\
                  "involving locally mapped foreign key columns for "\
                  "%s join condition "\
                  "'%s' on relationship %s." % (
                      primary and 'primary' or 'secondary',
                      join_condition,
                      self.prop
                  )
              err += \
                  "  Ensure that referencing columns are associated "\
                  "with a ForeignKey or ForeignKeyConstraint, or are "\
                  "annotated in the join condition with the foreign() "\
                  "annotation. To allow comparison operators other than "\
                  "'==', the relationship can be marked as viewonly=True."
  
              raise sa_exc.ArgumentError(err)
          else:
              err = "Could not locate any relevant foreign key columns "\
                  "for %s join condition '%s' on relationship %s." % (
                      primary and 'primary' or 'secondary',
                      join_condition,
                      self.prop
                  )
              err += \
                  '  Ensure that referencing columns are associated '\
                  'with a ForeignKey or ForeignKeyConstraint, or are '\
                  'annotated in the join condition with the foreign() '\
                  'annotation.'
              raise sa_exc.ArgumentError(err)
  
      def _determine_direction(self):
          """Determine if this relationship is one to many, many to one,
          many to many.
  
          """
          if self.secondaryjoin is not None:
              self.direction = MANYTOMANY
          else:
              parentcols = util.column_set(self.parent_selectable.c)
              targetcols = util.column_set(self.child_selectable.c)
  
              # fk collection which suggests ONETOMANY.
              onetomany_fk = targetcols.intersection(
                  self.foreign_key_columns)
  
              # fk collection which suggests MANYTOONE.
  
              manytoone_fk = parentcols.intersection(
                  self.foreign_key_columns)
  
              if onetomany_fk and manytoone_fk:
                  # fks on both sides.  test for overlap of local/remote
                  # with foreign key.
                  # we will gather columns directly from their annotations
                  # without deannotating, so that we can distinguish on a column
                  # that refers to itself.
  
                  # 1. columns that are both remote and FK suggest
                  # onetomany.
                  onetomany_local = self._gather_columns_with_annotation(
                      self.primaryjoin, "remote", "foreign")
  
                  # 2. columns that are FK but are not remote (e.g. local)
                  # suggest manytoone.
                  manytoone_local = set([c for c in
                                         self._gather_columns_with_annotation(
                                             self.primaryjoin,
                                             "foreign")
                                         if "remote" not in c._annotations])
  
                  # 3. if both collections are present, remove columns that
                  # refer to themselves.  This is for the case of
                  # and_(Me.id == Me.remote_id, Me.version == Me.version)
                  if onetomany_local and manytoone_local:
                      self_equated = self.remote_columns.intersection(
                          self.local_columns
                      )
                      onetomany_local = onetomany_local.difference(self_equated)
                      manytoone_local = manytoone_local.difference(self_equated)
  
                  # at this point, if only one or the other collection is
                  # present, we know the direction, otherwise it's still
                  # ambiguous.
  
                  if onetomany_local and not manytoone_local:
                      self.direction = ONETOMANY
                  elif manytoone_local and not onetomany_local:
                      self.direction = MANYTOONE
                  else:
                      raise sa_exc.ArgumentError(
                          "Can't determine relationship"
                          " direction for relationship '%s' - foreign "
                          "key columns within the join condition are present "
                          "in both the parent and the child's mapped tables.  "
                          "Ensure that only those columns referring "
                          "to a parent column are marked as foreign, "
                          "either via the foreign() annotation or "
                          "via the foreign_keys argument." % self.prop)
              elif onetomany_fk:
                  self.direction = ONETOMANY
              elif manytoone_fk:
                  self.direction = MANYTOONE
              else:
                  raise sa_exc.ArgumentError(
                      "Can't determine relationship "
                      "direction for relationship '%s' - foreign "
                      "key columns are present in neither the parent "
                      "nor the child's mapped tables" % self.prop)
  
      def _deannotate_pairs(self, collection):
          """provide deannotation for the various lists of
          pairs, so that using them in hashes doesn't incur
          high-overhead __eq__() comparisons against
          original columns mapped.
  
          """
          return [(x._deannotate(), y._deannotate())
                  for x, y in collection]
  
      def _setup_pairs(self):
          sync_pairs = []
          lrp = util.OrderedSet([])
          secondary_sync_pairs = []
  
          def go(joincond, collection):
              def visit_binary(binary, left, right):
                  if "remote" in right._annotations and \
                      "remote" not in left._annotations and \
                          self.can_be_synced_fn(left):
                      lrp.add((left, right))
                  elif "remote" in left._annotations and \
                      "remote" not in right._annotations and \
                          self.can_be_synced_fn(right):
                      lrp.add((right, left))
                  if binary.operator is operators.eq and \
                          self.can_be_synced_fn(left, right):
                      if "foreign" in right._annotations:
                          collection.append((left, right))
                      elif "foreign" in left._annotations:
                          collection.append((right, left))
              visit_binary_product(visit_binary, joincond)
  
          for joincond, collection in [
              (self.primaryjoin, sync_pairs),
              (self.secondaryjoin, secondary_sync_pairs)
          ]:
              if joincond is None:
                  continue
              go(joincond, collection)
  
          self.local_remote_pairs = self._deannotate_pairs(lrp)
          self.synchronize_pairs = self._deannotate_pairs(sync_pairs)
          self.secondary_synchronize_pairs = \
              self._deannotate_pairs(secondary_sync_pairs)
  
      @util.memoized_property
      def remote_columns(self):
          return self._gather_join_annotations("remote")
  
      @util.memoized_property
      def local_columns(self):
          return self._gather_join_annotations("local")
  
      @util.memoized_property
      def foreign_key_columns(self):
          return self._gather_join_annotations("foreign")
  
      @util.memoized_property
      def deannotated_primaryjoin(self):
          return _deep_deannotate(self.primaryjoin)
  
      @util.memoized_property
      def deannotated_secondaryjoin(self):
          if self.secondaryjoin is not None:
              return _deep_deannotate(self.secondaryjoin)
          else:
              return None
  
      def _gather_join_annotations(self, annotation):
          s = set(
              self._gather_columns_with_annotation(
                  self.primaryjoin, annotation)
          )
          if self.secondaryjoin is not None:
              s.update(
                  self._gather_columns_with_annotation(
                      self.secondaryjoin, annotation)
              )
          return set([x._deannotate() for x in s])
  
      def _gather_columns_with_annotation(self, clause, *annotation):
          annotation = set(annotation)
          return set([
              col for col in visitors.iterate(clause, {})
              if annotation.issubset(col._annotations)
          ])
  
      def join_targets(self, source_selectable,
                       dest_selectable,
                       aliased,
                       single_crit=None):
          """Given a source and destination selectable, create a
          join between them.
  
          This takes into account aliasing the join clause
          to reference the appropriate corresponding columns
          in the target objects, as well as the extra child
          criterion, equivalent column sets, etc.
  
          """
  
          # place a barrier on the destination such that
          # replacement traversals won't ever dig into it.
          # its internal structure remains fixed
          # regardless of context.
          dest_selectable = _shallow_annotate(
              dest_selectable,
              {'no_replacement_traverse': True})
  
          primaryjoin, secondaryjoin, secondary = self.primaryjoin, \
              self.secondaryjoin, self.secondary
  
          # adjust the join condition for single table inheritance,
          # in the case that the join is to a subclass
          # this is analogous to the
          # "_adjust_for_single_table_inheritance()" method in Query.
  
          if single_crit is not None:
              if secondaryjoin is not None:
                  secondaryjoin = secondaryjoin & single_crit
              else:
                  primaryjoin = primaryjoin & single_crit
  
          if aliased:
              if secondary is not None:
                  secondary = secondary.alias(flat=True)
                  primary_aliasizer = ClauseAdapter(secondary)
                  secondary_aliasizer = \
                      ClauseAdapter(dest_selectable,
                                    equivalents=self.child_equivalents).\
                      chain(primary_aliasizer)
                  if source_selectable is not None:
                      primary_aliasizer = \
                          ClauseAdapter(secondary).\
                          chain(ClauseAdapter(
                              source_selectable,
                              equivalents=self.parent_equivalents))
                  secondaryjoin = \
                      secondary_aliasizer.traverse(secondaryjoin)
              else:
                  primary_aliasizer = ClauseAdapter(
                      dest_selectable,
                      exclude_fn=_ColInAnnotations("local"),
                      equivalents=self.child_equivalents)
                  if source_selectable is not None:
                      primary_aliasizer.chain(
                          ClauseAdapter(source_selectable,
                                        exclude_fn=_ColInAnnotations("remote"),
                                        equivalents=self.parent_equivalents))
                  secondary_aliasizer = None
  
              primaryjoin = primary_aliasizer.traverse(primaryjoin)
              target_adapter = secondary_aliasizer or primary_aliasizer
              target_adapter.exclude_fn = None
          else:
              target_adapter = None
          return primaryjoin, secondaryjoin, secondary, \
              target_adapter, dest_selectable
  
      def create_lazy_clause(self, reverse_direction=False):
          binds = util.column_dict()
          lookup = collections.defaultdict(list)
          equated_columns = util.column_dict()
  
          if reverse_direction and self.secondaryjoin is None:
              for l, r in self.local_remote_pairs:
                  lookup[r].append((r, l))
                  equated_columns[l] = r
          else:
              # replace all "local side" columns, which is
              # anything that isn't marked "remote"
              for l, r in self.local_remote_pairs:
                  lookup[l].append((l, r))
                  equated_columns[r] = l
  
          def col_to_bind(col):
              if (reverse_direction and col in lookup) or \
                      (not reverse_direction and "local" in col._annotations):
                  if col in lookup:
                      for tobind, equated in lookup[col]:
                          if equated in binds:
                              return None
                  if col not in binds:
                      binds[col] = sql.bindparam(
                          None, None, type_=col.type, unique=True)
                  return binds[col]
              return None
  
          lazywhere = self.primaryjoin
          if self.secondaryjoin is None or not reverse_direction:
              lazywhere = visitors.replacement_traverse(
                  lazywhere, {}, col_to_bind)
  
          if self.secondaryjoin is not None:
              secondaryjoin = self.secondaryjoin
              if reverse_direction:
                  secondaryjoin = visitors.replacement_traverse(
                      secondaryjoin, {}, col_to_bind)
              lazywhere = sql.and_(lazywhere, secondaryjoin)
  
          bind_to_col = dict((binds[col].key, col) for col in binds)
  
          # this is probably not necessary
          lazywhere = _deep_deannotate(lazywhere)
  
          return lazywhere, bind_to_col, equated_columns
  
  
  class _ColInAnnotations(object):
      """Seralizable equivalent to:
  
          lambda c: "name" in c._annotations
      """
  
      def __init__(self, name):
          self.name = name
  
      def __call__(self, c):
          return self.name in c._annotations
  