sqlalchemy/orm/strategy_options.py

# Copyright (C) 2005-2016 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

    _merge_into_path = False
    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
                existing = path.entity_path[prop].get(
                    self.context, "path_with_polymorphic")
                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,
                        _existing_alias=existing)
                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 __str__(self):
        return "Load(strategy=%r)" % (self.strategy, )

    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._merge_into_path:
            # special helper for undefer_group
            existing = self.path.get(self.context, "loader")
            if existing:
                existing.local_opts.update(self.local_opts)
            else:
                self.path.set(self.context, "loader", self)

        elif 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 effective_path.is_token:
            for path in effective_path.generate_for_superclasses():
                if self._merge_into_path:
                    # special helper for undefer_group
                    existing = path.get(context, "loader")
                    if existing:
                        existing.local_opts.update(self.local_opts)
                    else:
                        path.set(context, "loader", loader)
                elif self._is_chain_link:
                    path.setdefault(context, "loader", loader)
                else:
                    path.set(context, "loader", loader)
        else:
            # only supported for the undefer_group() wildcard opt
            assert not self._merge_into_path
            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.")
        elif token.endswith(_DEFAULT_TOKEN):
            raiseerr = False

        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("addresses").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},
                               {"undefer_pks": 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))

    :param innerjoin: if ``True``, indicates that the joined eager load should
     use an inner join instead of the default of left outer join::

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

     In order to chain multiple eager joins together where some may be
     OUTER and others INNER, right-nested joins are used to link them::

        query(A).options(
            joinedload(A.bs, innerjoin=False).
                joinedload(B.cs, innerjoin=True)
        )

     The above query, linking A.bs via "outer" join and B.cs via "inner" join
     would render the joins as "a LEFT OUTER JOIN (b JOIN c)".   When using
     SQLite, this form of JOIN is translated to use full subqueries as this
     syntax is otherwise not directly supported.

     The ``innerjoin`` flag can also be stated with the term ``"unnested"``.
     This will prevent joins from being right-nested, and will instead
     link an "innerjoin" eagerload to an "outerjoin" eagerload by bypassing
     the "inner" join.   Using this form as follows::

        query(A).options(
            joinedload(A.bs, innerjoin=False).
                joinedload(B.cs, innerjoin="unnested")
        )

     Joins will be rendered as "a LEFT OUTER JOIN b LEFT OUTER JOIN c", so that
     all of "a" is matched rather than being incorrectly limited by a "b" that
     does not contain a "c".

     .. note:: The "unnested" flag does **not** affect the JOIN rendered
        from a many-to-many association table, e.g. a table configured
        as :paramref:`.relationship.secondary`, to the target table; for
        correctness of results, these joins are always INNER and are
        therefore right-nested if linked to an OUTER join.

     .. versionadded:: 0.9.4 Added support for "nesting" of eager "inner"
        joins.  See :ref:`feature_2976`.

     .. versionchanged:: 1.0.0 ``innerjoin=True`` now implies
        ``innerjoin="nested"``, whereas in 0.9 it implied
        ``innerjoin="unnested"``.  In order to achieve the pre-1.0 "unnested"
        inner join behavior, use the value ``innerjoin="unnested"``.
        See :ref:`migration_3008`.

    .. 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`

        :paramref:`.relationship.lazy`

        :paramref:`.relationship.innerjoin` - :func:`.relationship`-level
        version of the :paramref:`.joinedload.innerjoin` option.

    """
    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`

        :paramref:`.relationship.lazy`

    """
    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.

    .. seealso::

        :paramref:`.relationship.lazy`

    """
    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`

        :paramref:`.relationship.lazy`

    """
    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`

    """
    loadopt._merge_into_path = True
    return loadopt.set_column_strategy(
        "*",
        None,
        {"undefer_group_%s" % name: True}
    )


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