path: root/lib/sqlalchemy/orm/__init__.py

# mapper/__init__.py
# Copyright (C) 2005, 2006, 2007, 2008 Michael Bayer mike_mp@zzzcomputing.com
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

"""
Functional constructs for ORM configuration.

See the SQLAlchemy object relational tutorial and mapper configuration
documentation for an overview of how this module is used.
"""

from sqlalchemy.orm.mapper import Mapper, object_mapper, class_mapper, _mapper_registry
from sqlalchemy.orm.interfaces import MapperExtension, EXT_CONTINUE, EXT_STOP, EXT_PASS, ExtensionOption, PropComparator
from sqlalchemy.orm.properties import SynonymProperty, ComparableProperty, PropertyLoader, ColumnProperty, CompositeProperty, BackRef
from sqlalchemy.orm import mapper as mapperlib
from sqlalchemy.orm import strategies
from sqlalchemy.orm.query import Query, aliased
from sqlalchemy.orm.util import polymorphic_union, create_row_adapter
from sqlalchemy.orm.session import Session as _Session
from sqlalchemy.orm.session import object_session, sessionmaker
from sqlalchemy.orm.scoping import ScopedSession


__all__ = [ 'relation', 'column_property', 'composite', 'backref', 'eagerload',
            'eagerload_all', 'lazyload', 'noload', 'deferred', 'defer',
            'undefer', 'undefer_group', 'extension', 'mapper', 'clear_mappers',
            'compile_mappers', 'class_mapper', 'object_mapper', 'sessionmaker',
            'scoped_session', 'dynamic_loader', 'MapperExtension',
            'polymorphic_union', 'comparable_property',
            'create_session', 'synonym', 'contains_alias', 'Query', 'aliased',
            'contains_eager', 'EXT_CONTINUE', 'EXT_STOP', 'EXT_PASS',
            'object_session', 'PropComparator' ]


def scoped_session(session_factory, scopefunc=None):
  """Provides thread-local management of Sessions.

  This is a front-end function to the [sqlalchemy.orm.scoping#ScopedSession]
  class.

  Usage::

    Session = scoped_session(sessionmaker(autoflush=True))

  To instantiate a Session object which is part of the scoped
  context, instantiate normally::

    session = Session()

  Most session methods are available as classmethods from
  the scoped session::

    Session.commit()
    Session.close()

  To map classes so that new instances are saved in the current
  Session automatically, as well as to provide session-aware
  class attributes such as "query", use the `mapper` classmethod
  from the scoped session::

    mapper = Session.mapper
    mapper(Class, table, ...)

  """

  return ScopedSession(session_factory, scopefunc=scopefunc)

def create_session(bind=None, **kwargs):
    """create a new [sqlalchemy.orm.session#Session].

    The session by default does not begin a transaction, and requires that
    flush() be called explicitly in order to persist results to the database.

    It is recommended to use the [sqlalchemy.orm#sessionmaker()] function
    instead of create_session().
    """
    kwargs.setdefault('autoflush', False)
    kwargs.setdefault('transactional', False)
    return _Session(bind=bind, **kwargs)

def relation(argument, secondary=None, **kwargs):
    """Provide a relationship of a primary Mapper to a secondary Mapper.

    This corresponds to a parent-child or associative table relationship.
    The constructed class is an instance of [sqlalchemy.orm.properties#PropertyLoader].

      argument
          a class or Mapper instance, representing the target of the relation.

      secondary
        for a many-to-many relationship, specifies the intermediary table. The
        ``secondary`` keyword argument should generally only be used for a table
        that is not otherwise expressed in any class mapping. In particular,
        using the Association Object Pattern is
        generally mutually exclusive against using the ``secondary`` keyword
        argument.

      \**kwargs follow:

        association
          Deprecated; as of version 0.3.0 the association keyword is synonymous
          with applying the "all, delete-orphan" cascade to a "one-to-many"
          relationship. SA can now automatically reconcile a "delete" and
          "insert" operation of two objects with the same "identity" in a flush()
          operation into a single "update" statement, which is the pattern that
          "association" used to indicate.

        backref
          indicates the name of a property to be placed on the related mapper's
          class that will handle this relationship in the other direction,
          including synchronizing the object attributes on both sides of the
          relation. Can also point to a ``backref()`` construct for more
          configurability.

        cascade
          a string list of cascade rules which determines how persistence
          operations should be "cascaded" from parent to child.

        collection_class
          a class or function that returns a new list-holding object. will be
          used in place of a plain list for storing elements.

        foreign_keys
          a list of columns which are to be used as "foreign key" columns.
          this parameter should be used in conjunction with explicit
          ``primaryjoin`` and ``secondaryjoin`` (if needed) arguments, and the
          columns within the ``foreign_keys`` list should be present within
          those join conditions. Normally, ``relation()`` will inspect the
          columns within the join conditions to determine which columns are
          the "foreign key" columns, based on information in the ``Table``
          metadata. Use this argument when no ForeignKey's are present in the
          join condition, or to override the table-defined foreign keys.

        foreignkey
          deprecated. use the ``foreign_keys`` argument for foreign key
          specification, or ``remote_side`` for "directional" logic.

        join_depth=None
          when non-``None``, an integer value indicating how many levels
          deep eagerload joins should be constructed 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 loads will automatically stop chaining joins when they encounter
          a mapper which is already higher up in the chain.

        lazy=(True|False|None|'dynamic')
          specifies how the related items should be loaded. Values include:

            True - items should be loaded lazily when the property is first
                   accessed.

            False - items should be loaded "eagerly" in the same query as that
                    of the parent, using a JOIN or LEFT OUTER JOIN.

            None - 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' - a ``DynaLoader`` will be attached, which returns a
                        ``Query`` object for all read operations.  The dynamic-
                        collection supports only ``append()`` and ``remove()``
                        for write operations; changes to the dynamic property
                        will not be visible until the data is flushed to the
                        database.

        order_by
          indicates the ordering that should be applied when loading these items.

        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.

        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. Postgres, MySQL with InnoDB tables), ON UPDATE CASCADE is
          required for this operation.  The relation() 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 relation() 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).

        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
          ``post_update`` to "break" the cycle.

        primaryjoin
          a ClauseElement 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).

        private=False
          deprecated. setting ``private=True`` is the equivalent of setting
          ``cascade="all, delete-orphan"``, and indicates the lifecycle of child
          objects should be contained within that of the parent.

        remote_side
          used for self-referential relationships, indicates the column or list
          of columns that form the "remote side" of the relationship.

        secondaryjoin
          a ClauseElement 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.

        uselist=(True|False)
          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
          ``relation()``, 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 uselist to False.

        viewonly=False
          when set to True, the relation is used only for loading objects within
          the relationship, and has no effect on the unit-of-work flush process.
          Relations with viewonly can specify any kind of join conditions to
          provide additional views of related objects onto a parent object. Note
          that the functionality of a viewonly relationship has its limits -
          complicated join conditions may not compile into eager or lazy loaders
          properly. If this is the case, use an alternative method.

    """

    return PropertyLoader(argument, secondary=secondary, **kwargs)

def dynamic_loader(argument, secondary=None, primaryjoin=None, secondaryjoin=None, entity_name=None,
    foreign_keys=None, backref=None, post_update=False, cascade=None, remote_side=None, enable_typechecks=True,
    passive_deletes=False, order_by=None):
    """construct a dynamically-loading mapper property.

    This property is similar to relation(), except read operations
    return an active Query object, which reads from the database in all
    cases.  Items may be appended to the attribute via append(), or
    removed via remove(); changes will be persisted
    to the database during a flush().  However, no other list mutation
    operations are available.

    A subset of arguments available to relation() are available here.
    """

    from sqlalchemy.orm.dynamic import DynaLoader

    return PropertyLoader(argument, secondary=secondary, primaryjoin=primaryjoin,
            secondaryjoin=secondaryjoin, entity_name=entity_name, foreign_keys=foreign_keys, backref=backref,
            post_update=post_update, cascade=cascade, remote_side=remote_side, enable_typechecks=enable_typechecks,
            passive_deletes=passive_deletes, order_by=order_by,
            strategy_class=DynaLoader)

#def _relation_loader(mapper, secondary=None, primaryjoin=None, secondaryjoin=None, lazy=True, **kwargs):

def column_property(*args, **kwargs):
    """Provide a column-level property for use with a Mapper.

    Column-based properties can normally be applied to the mapper's
    ``properties`` dictionary using the ``schema.Column`` element directly.
    Use this function when the given column is not directly present within
    the mapper's selectable; examples include SQL expressions, functions,
    and scalar SELECT queries.

    Columns that aren't present in the mapper's selectable won't be persisted
    by the mapper and are effectively "read-only" attributes.

      \*cols
          list of Column objects to be mapped.

      group
          a group name for this property when marked as deferred.

      deferred
          when True, the column property is "deferred", meaning that
          it does not load immediately, and is instead loaded when the
          attribute is first accessed on an instance.  See also
          [sqlalchemy.orm#deferred()].

    """

    return ColumnProperty(*args, **kwargs)

def composite(class_, *cols, **kwargs):
    """Return a composite column-based property for use with a Mapper.

    This is very much like a column-based property except the given class
    is used to represent "composite" values composed of one or more columns.

    The class must implement a constructor with positional arguments matching
    the order of columns supplied here, as well as a __composite_values__()
    method which returns values in the same order.

    A simple example is representing separate two columns in a table as a
    single, first-class "Point" object::

      class Point(object):
          def __init__(self, x, y):
              self.x = x
              self.y = y
          def __composite_values__(self):
              return (self.x, self.y)

      # and then in the mapping:
      ... composite(Point, mytable.c.x, mytable.c.y) ...

    Arguments are:

    class\_
      The "composite type" class.

    \*cols
      List of Column objects to be mapped.

    group
      A group name for this property when marked as deferred.

    deferred
      When True, the column property is "deferred", meaning that
      it does not load immediately, and is instead loaded when the
      attribute is first accessed on an instance.  See also
      [sqlalchemy.orm#deferred()].

    comparator
      An optional instance of [sqlalchemy.orm#PropComparator] which
      provides SQL expression generation functions for this composite
      type.
    """

    return CompositeProperty(class_, *cols, **kwargs)


def backref(name, **kwargs):
    """Create a BackRef object with explicit arguments, which are the same arguments one
    can send to ``relation()``.

    Used with the `backref` keyword argument to ``relation()`` in
    place of a string argument.
    """

    return BackRef(name, **kwargs)

def deferred(*columns, **kwargs):
    """Return a ``DeferredColumnProperty``, which indicates this
    object attributes should only be loaded from its corresponding
    table column when first accessed.

    Used with the `properties` dictionary sent to ``mapper()``.
    """

    return ColumnProperty(deferred=True, *columns, **kwargs)

def mapper(class_, local_table=None, *args, **params):
    """Return a new [sqlalchemy.orm#Mapper] object.

      class\_
        The class to be mapped.

      local_table
        The table to which the class is mapped, or None if this
        mapper inherits from another mapper using concrete table
        inheritance.

      entity_name
        A name to be associated with the `class`, to allow alternate
        mappings for a single class.

      always_refresh
        If True, all query operations for this mapped class will
        overwrite all data within object instances that already
        exist within the session, erasing any in-memory changes with
        whatever information was loaded from the database.  Usage
        of this flag is highly discouraged; as an alternative,
        see the method `populate_existing()` on [sqlalchemy.orm.query#Query].

      allow_column_override
        If True, allows the usage of a ``relation()`` which has the
        same name as a column in the mapped table.  The table column
        will no longer be mapped.

      allow_null_pks
        Indicates that composite primary keys where one or more (but
        not all) columns contain NULL is a valid primary key.
        Primary keys which contain NULL values usually indicate that
        a result row does not contain an entity and should be
        skipped.

      batch
        Indicates that save operations of multiple entities can be
        batched together for efficiency.  setting to False indicates
        that an instance will be fully saved before saving the next
        instance, which includes inserting/updating all table rows
        corresponding to the entity as well as calling all
        ``MapperExtension`` methods corresponding to the save
        operation.

      column_prefix
        A string which will be prepended to the `key` name of all
        Columns when creating column-based properties from the given
        Table.  Does not affect explicitly specified column-based
        properties

      concrete
        If True, indicates this mapper should use concrete table
        inheritance with its parent mapper.

      extension
        A [sqlalchemy.orm#MapperExtension] instance or list of
        ``MapperExtension`` instances which will be applied to all
        operations by this ``Mapper``.

      inherits
        Another ``Mapper`` for which this ``Mapper`` will have an
        inheritance relationship with.

      inherit_condition
        For joined table inheritance, a SQL expression (constructed
        ``ClauseElement``) which will define how the two tables are
        joined; defaults to a natural join between the two tables.

      inherit_foreign_keys
        when inherit_condition is used and the condition contains no
        ForeignKey columns, specify the "foreign" columns of the join
        condition in this list.  else leave as None.

      order_by
        A single ``Column`` or list of ``Columns`` for which
        selection operations should use as the default ordering for
        entities.  Defaults to the OID/ROWID of the table if any, or
        the first primary key column of the table.

      non_primary
        Construct a ``Mapper`` that will define only the selection
        of instances, not their persistence.  Any number of non_primary
        mappers may be created for a particular class.

      polymorphic_on
        Used with mappers in an inheritance relationship, a ``Column``
        which will identify the class/mapper combination to be used
        with a particular row.  Requires the ``polymorphic_identity``
        value to be set for all mappers in the inheritance
        hierarchy.  The column specified by ``polymorphic_on`` is 
        usually a column that resides directly within the base 
        mapper's mapped table; alternatively, it may be a column
        that is only present within the <selectable> portion
        of the ``with_polymorphic`` argument.

      _polymorphic_map
        Used internally to propagate the full map of polymorphic
        identifiers to surrogate mappers.

      polymorphic_identity
        A value which will be stored in the Column denoted by
        polymorphic_on, corresponding to the *class identity* of
        this mapper.

      polymorphic_fetch
        specifies how subclasses mapped through joined-table
        inheritance will be fetched.  options are 'union',
        'select', and 'deferred'.  if the 'with_polymorphic' argument
        is present, defaults to 'union', otherwise defaults to
        'select'.

      properties
        A dictionary mapping the string names of object attributes
        to ``MapperProperty`` instances, which define the
        persistence behavior of that attribute.  Note that the
        columns in the mapped table are automatically converted into
        ``ColumnProperty`` instances based on the `key` property of
        each ``Column`` (although they can be overridden using this
        dictionary).

      include_properties
        An inclusive list of properties to map.  Columns present in the
        mapped table but not present in this list will not be automatically
        converted into properties.

      exclude_properties
        A list of properties not to map.  Columns present in the
        mapped table and present in this list will not be automatically
        converted into properties.  Note that neither this option nor
        include_properties will allow an end-run around Python inheritance.
        If mapped class ``B`` inherits from mapped class ``A``, no combination
        of includes or excludes will allow ``B`` to have fewer properties
        than its superclass, ``A``.

      primary_key
        A list of ``Column`` objects which define the *primary key*
        to be used against this mapper's selectable unit.  This is
        normally simply the primary key of the `local_table`, but
        can be overridden here.
    
      with_polymorphic
        A tuple in the form ``(<classes>, <selectable>)`` indicating the
        default style of "polymorphic" loading, that is, which tables
        are queried at once. <classes> is any single or list of mappers
        and/or classes indicating the inherited classes that should be
        loaded at once. The special value ``'*'`` may be used to indicate
        all descending classes should be loaded immediately. The second
        tuple argument <selectable> indicates a selectable that will be
        used to query for multiple classes. Normally, it is left as
        None, in which case this mapper will form an outer join from
        the base mapper's table to that of all desired sub-mappers.
        When specified, it provides the selectable to be used for
        polymorphic loading. When with_polymorphic includes mappers
        which load from a "concrete" inheriting table, the <selectable>
        argument is required, since it usually requires more complex
        UNION queries.
        
      select_table
        Deprecated.  Synonymous with 
        ``with_polymorphic=('*', <selectable>)``.

      version_id_col
        A ``Column`` which must have an integer type that will be
        used to keep a running *version id* of mapped entities in
        the database.  this is used during save operations to ensure
        that no other thread or process has updated the instance
        during the lifetime of the entity, else a
        ``ConcurrentModificationError`` exception is thrown.
    """

    return Mapper(class_, local_table, *args, **params)

def synonym(name, map_column=False, descriptor=None, proxy=False):
    """Set up `name` as a synonym to another mapped property.

    Used with the ``properties`` dictionary sent to  [sqlalchemy.orm#mapper()].

    Any existing attributes on the class which map the key name sent
    to the ``properties`` dictionary will be used by the synonym to
    provide instance-attribute behavior (that is, any Python property object,
    provided by the ``property`` builtin or providing a ``__get__()``,
    ``__set__()`` and ``__del__()`` method).  If no name exists for the key,
    the ``synonym()`` creates a default getter/setter object automatically
    and applies it to the class.

    `name` refers to the name of the existing mapped property, which
    can be any other ``MapperProperty`` including column-based
    properties and relations.

    If `map_column` is ``True``, an additional ``ColumnProperty`` is created
    on the mapper automatically, using the synonym's name as the keyname of
    the property, and the keyname of this ``synonym()`` as the name of the
    column to map.  For example, if a table has a column named ``status``::

        class MyClass(object):
            def _get_status(self):
                return self._status
            def _set_status(self, value):
                self._status = value
            status = property(_get_status, _set_status)

        mapper(MyClass, sometable, properties={
            "status":synonym("_status", map_column=True)
        })

    The column named ``status`` will be mapped to the attribute named
    ``_status``, and the ``status`` attribute on ``MyClass`` will be used to
    proxy access to the column-based attribute.

    The `proxy` keyword argument is deprecated and currently does nothing;
    synonyms now always establish an attribute getter/setter function if one
    is not already available.
    """

    return SynonymProperty(name, map_column=map_column, descriptor=descriptor)

def comparable_property(comparator_factory, descriptor=None):
    """Provide query semantics for an unmanaged attribute.

    Allows a regular Python @property (descriptor) to be used in Queries and
    SQL constructs like a managed attribute.  comparable_property wraps a
    descriptor with a proxy that directs operator overrides such as ==
    (__eq__) to the supplied comparator but proxies everything else through
    to the original descriptor::

      class MyClass(object):
          @property
          def myprop(self):
              return 'foo'

      class MyComparator(sqlalchemy.orm.interfaces.PropComparator):
          def __eq__(self, other):
              ....

      mapper(MyClass, mytable, properties=dict(
               'myprop': comparable_property(MyComparator)))

    Used with the ``properties`` dictionary sent to  [sqlalchemy.orm#mapper()].

    comparator_factory
      A PropComparator subclass or factory that defines operator behavior
      for this property.

    descriptor
      Optional when used in a ``properties={}`` declaration.  The Python
      descriptor or property to layer comparison behavior on top of.

      The like-named descriptor will be automatically retreived from the
      mapped class if left blank in a ``properties`` declaration.
    """
    return ComparableProperty(comparator_factory, descriptor)

def compile_mappers():
    """Compile all mappers that have been defined.

    This is equivalent to calling ``compile()`` on any individual mapper.
    """

    for m in list(_mapper_registry):
        m.compile()

def clear_mappers():
    """Remove all mappers that have been created thus far.

    The mapped classes will return to their initial "unmapped"
    state and can be re-mapped with new mappers.
    """
    mapperlib._COMPILE_MUTEX.acquire()
    try:
        for mapper in list(_mapper_registry):
            mapper.dispose()
    finally:
        mapperlib._COMPILE_MUTEX.release()

def extension(ext):
    """Return a ``MapperOption`` that will insert the given
    ``MapperExtension`` to the beginning of the list of extensions
    that will be called in the context of the ``Query``.

    Used with ``query.options()``.
    """

    return ExtensionOption(ext)

def eagerload(name, mapper=None):
    """Return a ``MapperOption`` that will convert the property of the given name into an eager load.

    Used with ``query.options()``.
    """

    return strategies.EagerLazyOption(name, lazy=False, mapper=mapper)

def eagerload_all(name, mapper=None):
    """Return a ``MapperOption`` that will convert all properties along the given dot-separated path into an eager load.

    For example, this::

        query.options(eagerload_all('orders.items.keywords'))...

    will set all of 'orders', 'orders.items', and 'orders.items.keywords'
    to load in one eager load.

    Used with ``query.options()``.
    """

    return strategies.EagerLazyOption(name, lazy=False, chained=True, mapper=mapper)

def lazyload(name, mapper=None):
    """Return a ``MapperOption`` that will convert the property of the
    given name into a lazy load.

    Used with ``query.options()``.
    """

    return strategies.EagerLazyOption(name, lazy=True, mapper=mapper)

def noload(name):
    """Return a ``MapperOption`` that will convert the property of the
    given name into a non-load.

    Used with ``query.options()``.
    """

    return strategies.EagerLazyOption(name, lazy=None)

def contains_alias(alias):
    """Return a ``MapperOption`` that will indicate to the query that
    the main table has been aliased.

    `alias` is the string name or ``Alias`` object representing the
    alias.
    """

    class AliasedRow(MapperExtension):
        def __init__(self, alias):
            self.alias = alias
            if isinstance(self.alias, basestring):
                self.translator = None
            else:
                self.translator = create_row_adapter(alias)
        
        def translate_row(self, mapper, context, row):
            if not self.translator:
                self.translator = create_row_adapter(mapper.mapped_table.alias(self.alias))
            return self.translator(row)

    return ExtensionOption(AliasedRow(alias))

def contains_eager(key, alias=None, decorator=None):
    """Return a ``MapperOption`` that will indicate to the query that
    the given attribute will be eagerly loaded.

    Used when feeding SQL result sets directly into
    ``query.instances()``.  Also bundles an ``EagerLazyOption`` to
    turn on eager loading in case it isn't already.

    `alias` is the string name of an alias, **or** an ``sql.Alias``
    object, which represents the aliased columns in the query.  This
    argument is optional.

    `decorator` is mutually exclusive of `alias` and is a
    row-processing function which will be applied to the incoming row
    before sending to the eager load handler.  use this for more
    sophisticated row adjustments beyond a straight alias.
    """

    return (strategies.EagerLazyOption(key, lazy=False), strategies.RowDecorateOption(key, alias=alias, decorator=decorator))

def defer(name):
    """Return a ``MapperOption`` that will convert the column property
    of the given name into a deferred load.

    Used with ``query.options()``"""
    return strategies.DeferredOption(name, defer=True)

def undefer(name):
    """Return a ``MapperOption`` that will convert the column property
    of the given name into a non-deferred (regular column) load.

    Used with ``query.options()``.
    """

    return strategies.DeferredOption(name, defer=False)

def undefer_group(name):
    """Return a ``MapperOption`` that will convert the given
    group of deferred column properties into a non-deferred (regular column) load.

    Used with ``query.options()``.
    """
    return strategies.UndeferGroupOption(name)