# orm/strategy_options.py
# Copyright (C) 2005-2013 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

"""

"""

from .interfaces import MapperOption, PropComparator
from .. import util
from ..sql.base import _generative, Generative
from .. import exc as sa_exc, inspect
from .base import _is_aliased_class, _class_to_mapper
from . import util as orm_util
from .path_registry import PathRegistry, TokenRegistry, \
        _WILDCARD_TOKEN, _DEFAULT_TOKEN

class Load(Generative, MapperOption):
    """Represents loader options which modify the state of a
    :class:`.Query` in order to affect how various mapped attributes are loaded.

    .. versionadded:: 0.9.0 The :meth:`.Load` system is a new foundation for
       the existing system of loader options, including options such as
       :func:`.orm.joinedload`, :func:`.orm.defer`, and others.   In particular,
       it introduces a new method-chained system that replaces the need for
       dot-separated paths as well as "_all()" options such as :func:`.orm.joinedload_all`.

    A :class:`.Load` object can be used directly or indirectly.  To use one
    directly, instantiate given the parent class.  This style of usage is
    useful when dealing with a :class:`.Query` that has multiple entities,
    or when producing a loader option that can be applied generically to
    any style of query::

        myopt = Load(MyClass).joinedload("widgets")

    The above ``myopt`` can now be used with :meth:`.Query.options`::

        session.query(MyClass).options(myopt)

    The :class:`.Load` construct is invoked indirectly whenever one makes use
    of the various loader options that are present in ``sqlalchemy.orm``, including
    options such as :func:`.orm.joinedload`, :func:`.orm.defer`, :func:`.orm.subqueryload`,
    and all the rest.  These constructs produce an "anonymous" form of the
    :class:`.Load` object which tracks attributes and options, but is not linked
    to a parent class until it is associated with a parent :class:`.Query`::

        # produce "unbound" Load object
        myopt = joinedload("widgets")

        # when applied using options(), the option is "bound" to the
        # class observed in the given query, e.g. MyClass
        session.query(MyClass).options(myopt)

    Whether the direct or indirect style is used, the :class:`.Load` object
    returned now represents a specific "path" along the entities of a :class:`.Query`.
    This path can be traversed using a standard method-chaining approach.
    Supposing a class hierarchy such as ``User``, ``User.addresses -> Address``,
    ``User.orders -> Order`` and ``Order.items -> Item``, we can specify a variety
    of loader options along each element in the "path"::

        session.query(User).options(
                    joinedload("addresses"),
                    subqueryload("orders").joinedload("items")
                )

    Where above, the ``addresses`` collection will be joined-loaded, the
    ``orders`` collection will be subquery-loaded, and within that subquery load
    the ``items`` collection will be joined-loaded.


    """
    def __init__(self, entity):
        insp = inspect(entity)
        self.path = insp._path_registry
        self.context = {}
        self.local_opts = {}

    def _generate(self):
        cloned = super(Load, self)._generate()
        cloned.local_opts = {}
        return cloned

    strategy = None
    propagate_to_loaders = False

    def process_query(self, query):
        self._process(query, True)

    def process_query_conditionally(self, query):
        self._process(query, False)

    def _process(self, query, raiseerr):
        current_path = query._current_path
        if current_path:
            for (token, start_path), loader in self.context.items():
                chopped_start_path = self._chop_path(start_path, current_path)
                if chopped_start_path is not None:
                    query._attributes[(token, chopped_start_path)] = loader
        else:
            query._attributes.update(self.context)

    def _generate_path(self, path, attr, wildcard_key, raiseerr=True):
        if raiseerr and not path.has_entity:
            if isinstance(path, TokenRegistry):
                raise sa_exc.ArgumentError(
                        "Wildcard token cannot be followed by another entity")
            else:
                raise sa_exc.ArgumentError(
                    "Attribute '%s' of entity '%s' does not "
                    "refer to a mapped entity" %
                    (path.prop.key, path.parent.entity)
                )

        if isinstance(attr, util.string_types):
            default_token = attr.endswith(_DEFAULT_TOKEN)
            if attr.endswith(_WILDCARD_TOKEN) or default_token:
                if default_token:
                    self.propagate_to_loaders = False
                if wildcard_key:
                    attr = "%s:%s" % (wildcard_key, attr)
                return path.token(attr)

            try:
                # use getattr on the class to work around
                # synonyms, hybrids, etc.
                attr = getattr(path.entity.class_, attr)
            except AttributeError:
                if raiseerr:
                    raise sa_exc.ArgumentError(
                        "Can't find property named '%s' on the "
                        "mapped entity %s in this Query. " % (
                            attr, path.entity)
                    )
                else:
                    return None
            else:
                attr = attr.property

            path = path[attr]
        else:
            prop = attr.property

            if not prop.parent.common_parent(path.mapper):
                if raiseerr:
                    raise sa_exc.ArgumentError("Attribute '%s' does not "
                            "link from element '%s'" % (attr, path.entity))
                else:
                    return None

            if getattr(attr, '_of_type', None):
                ac = attr._of_type
                ext_info = inspect(ac)

                path_element = ext_info.mapper
                if not ext_info.is_aliased_class:
                    ac = orm_util.with_polymorphic(
                                ext_info.mapper.base_mapper,
                                ext_info.mapper, aliased=True,
                                _use_mapper_path=True)
                path.entity_path[prop].set(self.context,
                                    "path_with_polymorphic", inspect(ac))
                path = path[prop][path_element]
            else:
                path = path[prop]

        if path.has_entity:
            path = path.entity_path
        return path

    def _coerce_strat(self, strategy):
        if strategy is not None:
            strategy = tuple(sorted(strategy.items()))
        return strategy

    @_generative
    def set_relationship_strategy(self, attr, strategy, propagate_to_loaders=True):
        strategy = self._coerce_strat(strategy)

        self.propagate_to_loaders = propagate_to_loaders
        # if the path is a wildcard, this will set propagate_to_loaders=False
        self.path = self._generate_path(self.path, attr, "relationship")
        self.strategy = strategy
        if strategy is not None:
            self._set_path_strategy()

    @_generative
    def set_column_strategy(self, attrs, strategy, opts=None):
        strategy = self._coerce_strat(strategy)

        for attr in attrs:
            path = self._generate_path(self.path, attr, "column")
            cloned = self._generate()
            cloned.strategy = strategy
            cloned.path = path
            cloned.propagate_to_loaders = True
            if opts:
                cloned.local_opts.update(opts)
            cloned._set_path_strategy()

    def _set_path_strategy(self):
        if self.path.has_entity:
            self.path.parent.set(self.context, "loader", self)
        else:
            self.path.set(self.context, "loader", self)

    def __getstate__(self):
        d = self.__dict__.copy()
        d["path"] = self.path.serialize()
        return d

    def __setstate__(self, state):
        self.__dict__.update(state)
        self.path = PathRegistry.deserialize(self.path)

    def _chop_path(self, to_chop, path):
        i = -1

        for i, (c_token, p_token) in enumerate(zip(to_chop, path.path)):
            if isinstance(c_token, util.string_types):
                # TODO: this is approximated from the _UnboundLoad
                # version and probably has issues, not fully covered.

                if i == 0 and c_token.endswith(':' + _DEFAULT_TOKEN):
                    return to_chop
                elif c_token != 'relationship:%s' % (_WILDCARD_TOKEN,) and c_token != p_token.key:
                    return None

            if c_token is p_token:
                continue
            else:
                return None
        return to_chop[i+1:]


class _UnboundLoad(Load):
    """Represent a loader option that isn't tied to a root entity.

    The loader option will produce an entity-linked :class:`.Load`
    object when it is passed :meth:`.Query.options`.

    This provides compatibility with the traditional system
    of freestanding options, e.g. ``joinedload('x.y.z')``.

    """
    def __init__(self):
        self.path = ()
        self._to_bind = set()
        self.local_opts = {}

    _is_chain_link = False

    def _set_path_strategy(self):
        self._to_bind.add(self)

    def _generate_path(self, path, attr, wildcard_key):
        if wildcard_key and isinstance(attr, util.string_types) and \
                attr in (_WILDCARD_TOKEN, _DEFAULT_TOKEN):
            if attr == _DEFAULT_TOKEN:
                self.propagate_to_loaders = False
            attr = "%s:%s" % (wildcard_key, attr)

        return path + (attr, )

    def __getstate__(self):
        d = self.__dict__.copy()
        d['path'] = ret = []
        for token in util.to_list(self.path):
            if isinstance(token, PropComparator):
                ret.append((token._parentmapper.class_, token.key))
            else:
                ret.append(token)
        return d

    def __setstate__(self, state):
        ret = []
        for key in state['path']:
            if isinstance(key, tuple):
                cls, propkey = key
                ret.append(getattr(cls, propkey))
            else:
                ret.append(key)
        state['path'] = tuple(ret)
        self.__dict__ = state

    def _process(self, query, raiseerr):
        for val in self._to_bind:
            val._bind_loader(query, query._attributes, raiseerr)

    @classmethod
    def _from_keys(self, meth, keys, chained, kw):
        opt = _UnboundLoad()

        def _split_key(key):
            if isinstance(key, util.string_types):
                # coerce fooload('*') into "default loader strategy"
                if key == _WILDCARD_TOKEN:
                    return (_DEFAULT_TOKEN, )
                # coerce fooload(".*") into "wildcard on default entity"
                elif key.startswith("." + _WILDCARD_TOKEN):
                    key = key[1:]
                return key.split(".")
            else:
                return (key,)
        all_tokens = [token for key in keys for token in _split_key(key)]

        for token in all_tokens[0:-1]:
            if chained:
                opt = meth(opt, token, **kw)
            else:
                opt = opt.defaultload(token)
            opt._is_chain_link = True

        opt = meth(opt, all_tokens[-1], **kw)
        opt._is_chain_link = False

        return opt


    def _chop_path(self, to_chop, path):
        i = -1
        for i, (c_token, (p_mapper, p_prop)) in enumerate(zip(to_chop, path.pairs())):
            if isinstance(c_token, util.string_types):
                if i == 0 and c_token.endswith(':' + _DEFAULT_TOKEN):
                    return to_chop
                elif c_token != 'relationship:%s' % (_WILDCARD_TOKEN,) and c_token != p_prop.key:
                    return None
            elif isinstance(c_token, PropComparator):
                if c_token.property is not p_prop:
                    return None
        else:
            i += 1

        return to_chop[i:]


    def _bind_loader(self, query, context, raiseerr):
        start_path = self.path
        # _current_path implies we're in a
        # secondary load with an existing path

        current_path = query._current_path
        if current_path:
            start_path = self._chop_path(start_path, current_path)

        if not start_path:
            return None

        token = start_path[0]
        if isinstance(token, util.string_types):
            entity = self._find_entity_basestring(query, token, raiseerr)
        elif isinstance(token, PropComparator):
            prop = token.property
            entity = self._find_entity_prop_comparator(
                                    query,
                                    prop.key,
                                    token._parententity,
                                    raiseerr)

        else:
            raise sa_exc.ArgumentError(
                    "mapper option expects "
                    "string key or list of attributes")

        if not entity:
            return

        path_element = entity.entity_zero

        # transfer our entity-less state into a Load() object
        # with a real entity path.
        loader = Load(path_element)
        loader.context = context
        loader.strategy = self.strategy

        path = loader.path
        for token in start_path:
            loader.path = path = loader._generate_path(
                                        loader.path, token, None, raiseerr)
            if path is None:
                return

        loader.local_opts.update(self.local_opts)

        if loader.path.has_entity:
            effective_path = loader.path.parent
        else:
            effective_path = loader.path

        # prioritize "first class" options over those
        # that were "links in the chain", e.g. "x" and "y" in someload("x.y.z")
        # versus someload("x") / someload("x.y")
        if self._is_chain_link:
            effective_path.setdefault(context, "loader", loader)
        else:
            effective_path.set(context, "loader", loader)

    def _find_entity_prop_comparator(self, query, token, mapper, raiseerr):
        if _is_aliased_class(mapper):
            searchfor = mapper
        else:
            searchfor = _class_to_mapper(mapper)
        for ent in query._mapper_entities:
            if ent.corresponds_to(searchfor):
                return ent
        else:
            if raiseerr:
                if not list(query._mapper_entities):
                    raise sa_exc.ArgumentError(
                        "Query has only expression-based entities - "
                        "can't find property named '%s'."
                         % (token, )
                    )
                else:
                    raise sa_exc.ArgumentError(
                        "Can't find property '%s' on any entity "
                        "specified in this Query.  Note the full path "
                        "from root (%s) to target entity must be specified."
                        % (token, ",".join(str(x) for
                            x in query._mapper_entities))
                    )
            else:
                return None

    def _find_entity_basestring(self, query, token, raiseerr):
        if token.endswith(':' + _WILDCARD_TOKEN):
            if len(list(query._mapper_entities)) != 1:
                if raiseerr:
                    raise sa_exc.ArgumentError(
                            "Wildcard loader can only be used with exactly "
                            "one entity.  Use Load(ent) to specify "
                            "specific entities.")

        for ent in query._mapper_entities:
            # return only the first _MapperEntity when searching
            # based on string prop name.   Ideally object
            # attributes are used to specify more exactly.
            return ent
        else:
            if raiseerr:
                raise sa_exc.ArgumentError(
                    "Query has only expression-based entities - "
                    "can't find property named '%s'."
                     % (token, )
                )
            else:
                return None



class loader_option(object):
    def __init__(self):
        pass

    def __call__(self, fn):
        self.name = name = fn.__name__
        self.fn = fn
        if hasattr(Load, name):
            raise TypeError("Load class already has a %s method." % (name))
        setattr(Load, name, fn)

        return self

    def _add_unbound_fn(self, fn):
        self._unbound_fn = fn
        fn_doc = self.fn.__doc__
        self.fn.__doc__ = """Produce a new :class:`.Load` object with the
:func:`.orm.%(name)s` option applied.

See :func:`.orm.%(name)s` for usage examples.

""" % {"name": self.name}

        fn.__doc__ = fn_doc
        return self

    def _add_unbound_all_fn(self, fn):
        self._unbound_all_fn = fn
        fn.__doc__ = """Produce a standalone "all" option for :func:`.orm.%(name)s`.

.. deprecated:: 0.9.0

    The "_all()" style is replaced by method chaining, e.g.::

        session.query(MyClass).options(
            %(name)s("someattribute").%(name)s("anotherattribute")
        )

""" % {"name": self.name}
        return self

@loader_option()
def contains_eager(loadopt, attr, alias=None):
    """Indicate that the given attribute should be eagerly loaded from
    columns stated manually in the query.

    This function is part of the :class:`.Load` interface and supports
    both method-chained and standalone operation.

    The option is used in conjunction with an explicit join that loads
    the desired rows, i.e.::

        sess.query(Order).\\
                join(Order.user).\\
                options(contains_eager(Order.user))

    The above query would join from the ``Order`` entity to its related
    ``User`` entity, and the returned ``Order`` objects would have the
    ``Order.user`` attribute pre-populated.

    :func:`contains_eager` also accepts an `alias` argument, which is the
    string name of an alias, an :func:`~sqlalchemy.sql.expression.alias`
    construct, or an :func:`~sqlalchemy.orm.aliased` construct. Use this when
    the eagerly-loaded rows are to come from an aliased table::

        user_alias = aliased(User)
        sess.query(Order).\\
                join((user_alias, Order.user)).\\
                options(contains_eager(Order.user, alias=user_alias))

    .. seealso::

        :ref:`contains_eager`

    """
    if alias is not None:
        if not isinstance(alias, str):
            info = inspect(alias)
            alias = info.selectable

    cloned = loadopt.set_relationship_strategy(
            attr,
            {"lazy": "joined"},
            propagate_to_loaders=False
        )
    cloned.local_opts['eager_from_alias'] = alias
    return cloned

@contains_eager._add_unbound_fn
def contains_eager(*keys, **kw):
    return _UnboundLoad()._from_keys(_UnboundLoad.contains_eager, keys, True, kw)

@loader_option()
def load_only(loadopt, *attrs):
    """Indicate that for a particular entity, only the given list
    of column-based attribute names should be loaded; all others will be
    deferred.

    This function is part of the :class:`.Load` interface and supports
    both method-chained and standalone operation.

    Example - given a class ``User``, load only the ``name`` and ``fullname``
    attributes::

        session.query(User).options(load_only("name", "fullname"))

    Example - given a relationship ``User.addresses -> Address``, specify
    subquery loading for the ``User.addresses`` collection, but on each ``Address``
    object load only the ``email_address`` attribute::

        session.query(User).options(
                subqueryload("addreses").load_only("email_address")
        )

    For a :class:`.Query` that has multiple entities, the lead entity can be
    specifically referred to using the :class:`.Load` constructor::

        session.query(User, Address).join(User.addresses).options(
                    Load(User).load_only("name", "fullname"),
                    Load(Address).load_only("email_addres")
                )


    .. versionadded:: 0.9.0

    """
    cloned = loadopt.set_column_strategy(
                attrs,
                {"deferred": False, "instrument": True}
            )
    cloned.set_column_strategy("*",
                    {"deferred": True, "instrument": True})
    return cloned

@load_only._add_unbound_fn
def load_only(*attrs):
    return _UnboundLoad().load_only(*attrs)

@loader_option()
def joinedload(loadopt, attr, innerjoin=None):
    """Indicate that the given attribute should be loaded using joined
    eager loading.

    This function is part of the :class:`.Load` interface and supports
    both method-chained and standalone operation.

    examples::

        # joined-load the "orders" collection on "User"
        query(User).options(joinedload(User.orders))

        # joined-load Order.items and then Item.keywords
        query(Order).options(joinedload(Order.items).joinedload(Item.keywords))

        # lazily load Order.items, but when Items are loaded,
        # joined-load the keywords collection
        query(Order).options(lazyload(Order.items).joinedload(Item.keywords))

    :func:`.orm.joinedload` also accepts a keyword argument `innerjoin=True` which
    indicates using an inner join instead of an outer::

        query(Order).options(joinedload(Order.user, innerjoin=True))

    .. note::

        The joins produced by :func:`.orm.joinedload` are **anonymously aliased**.
        The criteria by which the join proceeds cannot be modified, nor can the
        :class:`.Query` refer to these joins in any way, including ordering.

        To produce a specific SQL JOIN which is explicitly available, use
        :meth:`.Query.join`.   To combine explicit JOINs with eager loading
        of collections, use :func:`.orm.contains_eager`; see :ref:`contains_eager`.

    .. seealso::

        :ref:`loading_toplevel`

        :ref:`contains_eager`

        :func:`.orm.subqueryload`

        :func:`.orm.lazyload`

    """
    loader = loadopt.set_relationship_strategy(attr, {"lazy": "joined"})
    if innerjoin is not None:
        loader.local_opts['innerjoin'] = innerjoin
    return loader

@joinedload._add_unbound_fn
def joinedload(*keys, **kw):
    return _UnboundLoad._from_keys(
            _UnboundLoad.joinedload, keys, False, kw)

@joinedload._add_unbound_all_fn
def joinedload_all(*keys, **kw):
    return _UnboundLoad._from_keys(
            _UnboundLoad.joinedload, keys, True, kw)


@loader_option()
def subqueryload(loadopt, attr):
    """Indicate that the given attribute should be loaded using
    subquery eager loading.

    This function is part of the :class:`.Load` interface and supports
    both method-chained and standalone operation.

    examples::

        # subquery-load the "orders" collection on "User"
        query(User).options(subqueryload(User.orders))

        # subquery-load Order.items and then Item.keywords
        query(Order).options(subqueryload(Order.items).subqueryload(Item.keywords))

        # lazily load Order.items, but when Items are loaded,
        # subquery-load the keywords collection
        query(Order).options(lazyload(Order.items).subqueryload(Item.keywords))


    .. seealso::

        :ref:`loading_toplevel`

        :func:`.orm.joinedload`

        :func:`.orm.lazyload`

    """
    return loadopt.set_relationship_strategy(attr, {"lazy": "subquery"})

@subqueryload._add_unbound_fn
def subqueryload(*keys):
    return _UnboundLoad._from_keys(_UnboundLoad.subqueryload, keys, False, {})

@subqueryload._add_unbound_all_fn
def subqueryload_all(*keys):
    return _UnboundLoad._from_keys(_UnboundLoad.subqueryload, keys, True, {})

@loader_option()
def lazyload(loadopt, attr):
    """Indicate that the given attribute should be loaded using "lazy"
    loading.

    This function is part of the :class:`.Load` interface and supports
    both method-chained and standalone operation.

    """
    return loadopt.set_relationship_strategy(attr, {"lazy": "select"})

@lazyload._add_unbound_fn
def lazyload(*keys):
    return _UnboundLoad._from_keys(_UnboundLoad.lazyload, keys, False, {})

@lazyload._add_unbound_all_fn
def lazyload_all(*keys):
    return _UnboundLoad._from_keys(_UnboundLoad.lazyload, keys, True, {})

@loader_option()
def immediateload(loadopt, attr):
    """Indicate that the given attribute should be loaded using
    an immediate load with a per-attribute SELECT statement.

    This function is part of the :class:`.Load` interface and supports
    both method-chained and standalone operation.

    .. seealso::

        :ref:`loading_toplevel`

        :func:`.orm.joinedload`

        :func:`.orm.lazyload`

    """
    loader = loadopt.set_relationship_strategy(attr, {"lazy": "immediate"})
    return loader

@immediateload._add_unbound_fn
def immediateload(*keys):
    return _UnboundLoad._from_keys(_UnboundLoad.immediateload, keys, False, {})


@loader_option()
def noload(loadopt, attr):
    """Indicate that the given relationship attribute should remain unloaded.

    This function is part of the :class:`.Load` interface and supports
    both method-chained and standalone operation.

    :func:`.orm.noload` applies to :func:`.relationship` attributes; for
    column-based attributes, see :func:`.orm.defer`.

    """

    return loadopt.set_relationship_strategy(attr, {"lazy": "noload"})

@noload._add_unbound_fn
def noload(*keys):
    return _UnboundLoad._from_keys(_UnboundLoad.noload, keys, False, {})

@loader_option()
def defaultload(loadopt, attr):
    """Indicate an attribute should load using its default loader style.

    This method is used to link to other loader options, such as
    to set the :func:`.orm.defer` option on a class that is linked to
    a relationship of the parent class being loaded, :func:`.orm.defaultload`
    can be used to navigate this path without changing the loading style
    of the relationship::

        session.query(MyClass).options(defaultload("someattr").defer("some_column"))

    .. seealso::

        :func:`.orm.defer`

        :func:`.orm.undefer`

    """
    return loadopt.set_relationship_strategy(
                attr,
                None
            )

@defaultload._add_unbound_fn
def defaultload(*keys):
    return _UnboundLoad._from_keys(_UnboundLoad.defaultload, keys, False, {})

@loader_option()
def defer(loadopt, key):
    """Indicate that the given column-oriented attribute should be deferred, e.g.
    not loaded until accessed.

    This function is part of the :class:`.Load` interface and supports
    both method-chained and standalone operation.

    e.g.::

        from sqlalchemy.orm import defer

        session.query(MyClass).options(
                            defer("attribute_one"),
                            defer("attribute_two"))

        session.query(MyClass).options(
                            defer(MyClass.attribute_one),
                            defer(MyClass.attribute_two))

    To specify a deferred load of an attribute on a related class,
    the path can be specified one token at a time, specifying the loading
    style for each link along the chain.  To leave the loading style
    for a link unchanged, use :func:`.orm.defaultload`::

        session.query(MyClass).options(defaultload("someattr").defer("some_column"))

    A :class:`.Load` object that is present on a certain path can have
    :meth:`.Load.defer` called multiple times, each will operate on the same
    parent entity::


        session.query(MyClass).options(
                        defaultload("someattr").
                            defer("some_column").
                            defer("some_other_column").
                            defer("another_column")
            )

    :param key: Attribute to be deferred.

    :param \*addl_attrs: Deprecated; this option supports the old 0.8 style
     of specifying a path as a series of attributes, which is now superseded
     by the method-chained style.

    .. seealso::

        :ref:`deferred`

        :func:`.orm.undefer`

    """
    return loadopt.set_column_strategy(
                (key, ),
                {"deferred": True, "instrument": True}
            )


@defer._add_unbound_fn
def defer(key, *addl_attrs):
    return _UnboundLoad._from_keys(_UnboundLoad.defer, (key, ) + addl_attrs, False, {})

@loader_option()
def undefer(loadopt, key):
    """Indicate that the given column-oriented attribute should be undeferred, e.g.
    specified within the SELECT statement of the entity as a whole.

    The column being undeferred is typically set up on the mapping as a
    :func:`.deferred` attribute.

    This function is part of the :class:`.Load` interface and supports
    both method-chained and standalone operation.

    Examples::

        # undefer two columns
        session.query(MyClass).options(undefer("col1"), undefer("col2"))

        # undefer all columns specific to a single class using Load + *
        session.query(MyClass, MyOtherClass).options(Load(MyClass).undefer("*"))

    :param key: Attribute to be undeferred.

    :param \*addl_attrs: Deprecated; this option supports the old 0.8 style
     of specifying a path as a series of attributes, which is now superseded
     by the method-chained style.

    .. seealso::

        :ref:`deferred`

        :func:`.orm.defer`

        :func:`.orm.undefer_group`

    """
    return loadopt.set_column_strategy(
                (key, ),
                {"deferred": False, "instrument": True}
            )

@undefer._add_unbound_fn
def undefer(key, *addl_attrs):
    return _UnboundLoad._from_keys(_UnboundLoad.undefer, (key, ) + addl_attrs, False, {})

@loader_option()
def undefer_group(loadopt, name):
    """Indicate that columns within the given deferred group name should be undeferred.

    The columns being undeferred are set up on the mapping as
    :func:`.deferred` attributes and include a "group" name.

    E.g::

        session.query(MyClass).options(undefer_group("large_attrs"))

    To undefer a group of attributes on a related entity, the path can be
    spelled out using relationship loader options, such as :func:`.orm.defaultload`::

        session.query(MyClass).options(defaultload("someattr").undefer_group("large_attrs"))

    .. versionchanged:: 0.9.0 :func:`.orm.undefer_group` is now specific to a
       particiular entity load path.

    .. seealso::

        :ref:`deferred`

        :func:`.orm.defer`

        :func:`.orm.undefer`

    """
    return loadopt.set_column_strategy(
                            "*",
                            None,
                            {"undefer_group": name}
                    )

@undefer_group._add_unbound_fn
def undefer_group(name):
    return _UnboundLoad().undefer_group(name)

