# orm/strategies.py
# Copyright (C) 2005-2011 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

"""sqlalchemy.orm.interfaces.LoaderStrategy 
   implementations, and related MapperOptions."""

from sqlalchemy import exc as sa_exc
from sqlalchemy import sql, util, log, event
from sqlalchemy.sql import util as sql_util
from sqlalchemy.sql import visitors, expression, operators
from sqlalchemy.orm import mapper, attributes, interfaces, exc as orm_exc
from sqlalchemy.orm.mapper import _none_set
from sqlalchemy.orm.interfaces import (
    LoaderStrategy, StrategizedOption, MapperOption, PropertyOption,
    serialize_path, deserialize_path, StrategizedProperty
    )
from sqlalchemy.orm import session as sessionlib, unitofwork
from sqlalchemy.orm import util as mapperutil
from sqlalchemy.orm.query import Query
import itertools

def _register_attribute(strategy, mapper, useobject,
        compare_function=None, 
        typecallable=None,
        copy_function=None, 
        mutable_scalars=False, 
        uselist=False,
        callable_=None, 
        proxy_property=None, 
        active_history=False,
        impl_class=None, 
        **kw
):

    prop = strategy.parent_property

    attribute_ext = list(util.to_list(prop.extension, default=[]))

    listen_hooks = []

    if useobject and prop.single_parent:
        listen_hooks.append(single_parent_validator)

    if prop.key in prop.parent._validators:
        listen_hooks.append(
            lambda desc, prop: mapperutil._validator_events(desc, 
                                prop.key, 
                                prop.parent._validators[prop.key])
        )

    if useobject:
        listen_hooks.append(unitofwork.track_cascade_events)

    # need to assemble backref listeners
    # after the singleparentvalidator, mapper validator
    backref = kw.pop('backref', None)
    if backref:
        listen_hooks.append(
            lambda desc, prop: attributes.backref_listeners(desc, 
                                backref, 
                                uselist)
        )

    for m in mapper.self_and_descendants:
        if prop is m._props.get(prop.key):

            desc = attributes.register_attribute_impl(
                m.class_, 
                prop.key, 
                parent_token=prop,
                mutable_scalars=mutable_scalars,
                uselist=uselist, 
                copy_function=copy_function, 
                compare_function=compare_function, 
                useobject=useobject,
                extension=attribute_ext, 
                trackparent=useobject, 
                typecallable=typecallable,
                callable_=callable_, 
                active_history=active_history,
                impl_class=impl_class,
                doc=prop.doc,
                **kw
                )

            for hook in listen_hooks:
                hook(desc, prop)

class UninstrumentedColumnLoader(LoaderStrategy):
    """Represent the a non-instrumented MapperProperty.

    The polymorphic_on argument of mapper() often results in this,
    if the argument is against the with_polymorphic selectable.

    """
    def init(self):
        self.columns = self.parent_property.columns

    def setup_query(self, context, entity, path, reduced_path, adapter, 
                            column_collection=None, **kwargs):
        for c in self.columns:
            if adapter:
                c = adapter.columns[c]
            column_collection.append(c)

    def create_row_processor(self, selectcontext, path, reduced_path, mapper, row, adapter):
        return None, None, None

class ColumnLoader(LoaderStrategy):
    """Strategize the loading of a plain column-based MapperProperty."""

    def init(self):
        self.columns = self.parent_property.columns
        self.is_composite = hasattr(self.parent_property, 'composite_class')

    def setup_query(self, context, entity, path, reduced_path, adapter, 
                            column_collection=None, **kwargs):
        for c in self.columns:
            if adapter:
                c = adapter.columns[c]
            column_collection.append(c)

    def init_class_attribute(self, mapper):
        self.is_class_level = True
        coltype = self.columns[0].type
        # TODO: check all columns ?  check for foreign key as well?
        active_history = self.parent_property.active_history or \
                            self.columns[0].primary_key

        _register_attribute(self, mapper, useobject=False,
            compare_function=coltype.compare_values,
            copy_function=coltype.copy_value,
            mutable_scalars=self.columns[0].type.is_mutable(),
            active_history = active_history
       )

    def create_row_processor(self, selectcontext, path, reduced_path, mapper, row, adapter):
        key = self.key
        # look through list of columns represented here
        # to see which, if any, is present in the row.
        for col in self.columns:
            if adapter:
                col = adapter.columns[col]
            if col is not None and col in row:
                def new_execute(state, dict_, row):
                    dict_[key] = row[col]
                return new_execute, None, None
        else:
            def new_execute(state, dict_, row):
                state.expire_attribute_pre_commit(dict_, key)
            return new_execute, None, None

log.class_logger(ColumnLoader)

class DeferredColumnLoader(LoaderStrategy):
    """Strategize the loading of a deferred column-based MapperProperty."""

    def create_row_processor(self, selectcontext, path, reduced_path, mapper, row, adapter):
        col = self.columns[0]
        if adapter:
            col = adapter.columns[col]

        key = self.key
        if col in row:
            return self.parent_property._get_strategy(ColumnLoader).\
                        create_row_processor(
                                selectcontext, path, reduced_path, mapper, row, adapter)

        elif not self.is_class_level:
            def new_execute(state, dict_, row):
                state.set_callable(dict_, key, LoadDeferredColumns(state, key))
        else:
            def new_execute(state, dict_, row):
                # reset state on the key so that deferred callables
                # fire off on next access.
                state.reset(dict_, key)

        return new_execute, None, None

    def init(self):
        if hasattr(self.parent_property, 'composite_class'):
            raise NotImplementedError("Deferred loading for composite "
                                    "types not implemented yet")
        self.columns = self.parent_property.columns
        self.group = self.parent_property.group

    def init_class_attribute(self, mapper):
        self.is_class_level = True

        _register_attribute(self, mapper, useobject=False,
             compare_function=self.columns[0].type.compare_values,
             copy_function=self.columns[0].type.copy_value,
             mutable_scalars=self.columns[0].type.is_mutable(),
             callable_=self._load_for_state,
             expire_missing=False
        )

    def setup_query(self, context, entity, path, reduced_path, adapter, 
                                only_load_props=None, **kwargs):
        if (
                self.group is not None and 
                context.attributes.get(('undefer', self.group), False)
            ) or (only_load_props and self.key in only_load_props):
            self.parent_property._get_strategy(ColumnLoader).\
                            setup_query(context, entity,
                                        path, reduced_path, adapter, **kwargs)

    def _load_for_state(self, state, passive):
        if not state.key:
            return attributes.ATTR_EMPTY

        if passive is attributes.PASSIVE_NO_FETCH:
            return attributes.PASSIVE_NO_RESULT

        prop = self.parent_property
        localparent = state.manager.mapper

        if self.group:
            toload = [
                    p.key for p in 
                    localparent.iterate_properties 
                    if isinstance(p, StrategizedProperty) and 
                      isinstance(p.strategy, DeferredColumnLoader) and 
                      p.group==self.group
                    ]
        else:
            toload = [self.key]

        # narrow the keys down to just those which have no history
        group = [k for k in toload if k in state.unmodified]

        session = sessionlib._state_session(state)
        if session is None:
            raise orm_exc.DetachedInstanceError(
                "Parent instance %s is not bound to a Session; "
                "deferred load operation of attribute '%s' cannot proceed" % 
                (mapperutil.state_str(state), self.key)
                )

        query = session.query(localparent)
        query._load_on_ident(state.key, 
                    only_load_props=group, refresh_state=state)
        return attributes.ATTR_WAS_SET

log.class_logger(DeferredColumnLoader)

class LoadDeferredColumns(object):
    """serializable loader object used by DeferredColumnLoader"""

    def __init__(self, state, key):
        self.state = state
        self.key = key

    def __call__(self, passive=attributes.PASSIVE_OFF):
        state, key = self.state, self.key

        localparent = state.manager.mapper
        prop = localparent._props[key]
        strategy = prop._strategies[DeferredColumnLoader]
        return strategy._load_for_state(state, passive)

class DeferredOption(StrategizedOption):
    propagate_to_loaders = True

    def __init__(self, key, defer=False):
        super(DeferredOption, self).__init__(key)
        self.defer = defer

    def get_strategy_class(self):
        if self.defer:
            return DeferredColumnLoader
        else:
            return ColumnLoader

class UndeferGroupOption(MapperOption):
    propagate_to_loaders = True

    def __init__(self, group):
        self.group = group

    def process_query(self, query):
        query._attributes[('undefer', self.group)] = True

class AbstractRelationshipLoader(LoaderStrategy):
    """LoaderStratgies which deal with related objects."""

    def init(self):
        self.mapper = self.parent_property.mapper
        self.target = self.parent_property.target
        self.table = self.parent_property.table
        self.uselist = self.parent_property.uselist

class NoLoader(AbstractRelationshipLoader):
    """Strategize a relationship() that doesn't load data automatically."""

    def init_class_attribute(self, mapper):
        self.is_class_level = True

        _register_attribute(self, mapper,
            useobject=True, 
            uselist=self.parent_property.uselist,
            typecallable = self.parent_property.collection_class,
        )

    def create_row_processor(self, selectcontext, path, reduced_path, mapper, row, adapter):
        def new_execute(state, dict_, row):
            state.initialize(self.key)
        return new_execute, None, None

log.class_logger(NoLoader)

class LazyLoader(AbstractRelationshipLoader):
    """Strategize a relationship() that loads when first accessed."""

    def init(self):
        super(LazyLoader, self).init()
        self.__lazywhere, \
        self.__bind_to_col, \
        self._equated_columns = self._create_lazy_clause(self.parent_property)

        self.logger.info("%s lazy loading clause %s", self, self.__lazywhere)

        # determine if our "lazywhere" clause is the same as the mapper's
        # get() clause.  then we can just use mapper.get()
        #from sqlalchemy.orm import query
        self.use_get = not self.uselist and \
                        self.mapper._get_clause[0].compare(
                            self.__lazywhere, 
                            use_proxies=True, 
                            equivalents=self.mapper._equivalent_columns
                        )

        if self.use_get:
            for col in self._equated_columns.keys():
                if col in self.mapper._equivalent_columns:
                    for c in self.mapper._equivalent_columns[col]:
                        self._equated_columns[c] = self._equated_columns[col]

            self.logger.info("%s will use query.get() to "
                                    "optimize instance loads" % self)

    def init_class_attribute(self, mapper):
        self.is_class_level = True

        # MANYTOONE currently only needs the 
        # "old" value for delete-orphan
        # cascades.  the required _SingleParentValidator 
        # will enable active_history
        # in that case.  otherwise we don't need the 
        # "old" value during backref operations.
        _register_attribute(self, 
                mapper,
                useobject=True,
                callable_=self._load_for_state,
                uselist = self.parent_property.uselist,
                backref = self.parent_property.back_populates,
                typecallable = self.parent_property.collection_class,
                active_history = \
                    self.parent_property.active_history or \
                    self.parent_property.direction is not \
                        interfaces.MANYTOONE or \
                    not self.use_get,
                )

    def lazy_clause(self, state, reverse_direction=False, 
                                alias_secondary=False, 
                                adapt_source=None):
        if state is None:
            return self._lazy_none_clause(
                                        reverse_direction, 
                                        adapt_source=adapt_source)

        if not reverse_direction:
            criterion, bind_to_col, rev = \
                                            self.__lazywhere, \
                                            self.__bind_to_col, \
                                            self._equated_columns
        else:
            criterion, bind_to_col, rev = \
                                LazyLoader._create_lazy_clause(
                                        self.parent_property,
                                        reverse_direction=reverse_direction)

        if reverse_direction:
            mapper = self.parent_property.mapper
        else:
            mapper = self.parent_property.parent

        o = state.obj() # strong ref
        dict_ = attributes.instance_dict(o)

        # use the "committed state" only if we're in a flush
        # for this state.

        sess = sessionlib._state_session(state)
        if sess is not None and sess._flushing:
            def visit_bindparam(bindparam):
                if bindparam.key in bind_to_col:
                    bindparam.callable = \
                                lambda: mapper._get_committed_state_attr_by_column(
                                        state, dict_, bind_to_col[bindparam.key])
        else:
            def visit_bindparam(bindparam):
                if bindparam.key in bind_to_col:
                    bindparam.callable = lambda: mapper._get_state_attr_by_column(
                                            state, dict_, bind_to_col[bindparam.key])


        if self.parent_property.secondary is not None and alias_secondary:
            criterion = sql_util.ClauseAdapter(
                                self.parent_property.secondary.alias()).\
                                traverse(criterion)

        criterion = visitors.cloned_traverse(
                                criterion, {}, {'bindparam':visit_bindparam})

        if adapt_source:
            criterion = adapt_source(criterion)
        return criterion

    def _lazy_none_clause(self, reverse_direction=False, adapt_source=None):
        if not reverse_direction:
            criterion, bind_to_col, rev = \
                                        self.__lazywhere, \
                                        self.__bind_to_col,\
                                        self._equated_columns
        else:
            criterion, bind_to_col, rev = \
                                LazyLoader._create_lazy_clause(
                                    self.parent_property,
                                    reverse_direction=reverse_direction)

        criterion = sql_util.adapt_criterion_to_null(criterion, bind_to_col)

        if adapt_source:
            criterion = adapt_source(criterion)
        return criterion

    def _load_for_state(self, state, passive):
        if not state.key and \
            (not self.parent_property.load_on_pending or not state.session_id):
            return attributes.ATTR_EMPTY

        instance_mapper = state.manager.mapper
        prop = self.parent_property
        key = self.key
        prop_mapper = self.mapper
        pending = not state.key

        if (
                (passive is attributes.PASSIVE_NO_FETCH or \
                    passive is attributes.PASSIVE_NO_FETCH_RELATED) and 
                not self.use_get
            ) or (
                passive is attributes.PASSIVE_ONLY_PERSISTENT and 
                pending
            ):
            return attributes.PASSIVE_NO_RESULT

        session = sessionlib._state_session(state)
        if not session:
            raise orm_exc.DetachedInstanceError(
                "Parent instance %s is not bound to a Session; "
                "lazy load operation of attribute '%s' cannot proceed" % 
                (mapperutil.state_str(state), key)
            )

        # if we have a simple primary key load, check the 
        # identity map without generating a Query at all
        if self.use_get:
            if session._flushing:
                get_attr = instance_mapper._get_committed_state_attr_by_column
            else:
                get_attr = instance_mapper._get_state_attr_by_column

            dict_ = state.dict
            if passive is attributes.PASSIVE_NO_FETCH_RELATED:
                attr_passive = attributes.PASSIVE_OFF
            else:
                attr_passive = passive

            ident = [
                get_attr(
                        state,
                        state.dict,
                        self._equated_columns[pk],
                        passive=attr_passive)
                for pk in prop_mapper.primary_key
            ]
            if attributes.PASSIVE_NO_RESULT in ident:
                return attributes.PASSIVE_NO_RESULT

            if _none_set.issuperset(ident):
                return None

            ident_key = prop_mapper.identity_key_from_primary_key(ident)
            instance = Query._get_from_identity(session, ident_key, passive)
            if instance is not None:
                return instance
            elif passive is attributes.PASSIVE_NO_FETCH or \
                passive is attributes.PASSIVE_NO_FETCH_RELATED:
                return attributes.PASSIVE_NO_RESULT

        q = session.query(prop_mapper)._adapt_all_clauses()

        # don't autoflush on pending
        if pending:
            q = q.autoflush(False)

        if state.load_path:
            q = q._with_current_path(state.load_path + (key,))

        if state.load_options:
            q = q._conditional_options(*state.load_options)

        if self.use_get:
            return q._load_on_ident(ident_key)

        if prop.order_by:
            q = q.order_by(*util.to_list(prop.order_by))

        for rev in prop._reverse_property:
            # reverse props that are MANYTOONE are loading *this* 
            # object from get(), so don't need to eager out to those.
            if rev.direction is interfaces.MANYTOONE and \
                        rev._use_get and \
                        not isinstance(rev.strategy, LazyLoader):
                q = q.options(EagerLazyOption((rev.key,), lazy='select'))

        lazy_clause = self.lazy_clause(state)

        if pending:
            bind_values = sql_util.bind_values(lazy_clause)
            if None in bind_values:
                return None

        q = q.filter(lazy_clause)

        result = q.all()
        if self.uselist:
            return result
        else:
            l = len(result)
            if l:
                if l > 1:
                    util.warn(
                        "Multiple rows returned with "
                        "uselist=False for lazily-loaded attribute '%s' " 
                        % prop)

                return result[0]
            else:
                return None

    def create_row_processor(self, selectcontext, path, reduced_path, 
                                    mapper, row, adapter):
        key = self.key
        if not self.is_class_level:
            def new_execute(state, dict_, row):
                # we are not the primary manager for this attribute 
                # on this class - set up a
                # per-instance lazyloader, which will override the 
                # class-level behavior.
                # this currently only happens when using a 
                # "lazyload" option on a "no load"
                # attribute - "eager" attributes always have a 
                # class-level lazyloader installed.
                state.set_callable(dict_, key, LoadLazyAttribute(state, key))
        else:
            def new_execute(state, dict_, row):
                # we are the primary manager for this attribute on 
                # this class - reset its
                # per-instance attribute state, so that the class-level 
                # lazy loader is
                # executed when next referenced on this instance.
                # this is needed in
                # populate_existing() types of scenarios to reset 
                # any existing state.
                state.reset(dict_, key)

        return new_execute, None, None

    @classmethod
    def _create_lazy_clause(cls, prop, reverse_direction=False):
        binds = util.column_dict()
        lookup = util.column_dict()
        equated_columns = util.column_dict()

        if reverse_direction and prop.secondaryjoin is None:
            for l, r in prop.local_remote_pairs:
                _list = lookup.setdefault(r, [])
                _list.append((r, l))
                equated_columns[l] = r
        else:
            for l, r in prop.local_remote_pairs:
                _list = lookup.setdefault(l, [])
                _list.append((l, r))
                equated_columns[r] = l

        def col_to_bind(col):
            if col in lookup:
                for tobind, equated in lookup[col]:
                    if equated in binds:
                        return None
                if col not in binds:
                    binds[col] = sql.bindparam(None, None, type_=col.type)
                return binds[col]
            return None

        lazywhere = prop.primaryjoin

        if prop.secondaryjoin is None or not reverse_direction:
            lazywhere = visitors.replacement_traverse(
                                            lazywhere, {}, col_to_bind) 

        if prop.secondaryjoin is not None:
            secondaryjoin = prop.secondaryjoin
            if reverse_direction:
                secondaryjoin = visitors.replacement_traverse(
                                            secondaryjoin, {}, col_to_bind)
            lazywhere = sql.and_(lazywhere, secondaryjoin)

        bind_to_col = dict((binds[col].key, col) for col in binds)

        return lazywhere, bind_to_col, equated_columns

log.class_logger(LazyLoader)

class LoadLazyAttribute(object):
    """serializable loader object used by LazyLoader"""

    def __init__(self, state, key):
        self.state = state
        self.key = key

    def __call__(self, passive=attributes.PASSIVE_OFF):
        state, key = self.state, self.key
        instance_mapper = state.manager.mapper
        prop = instance_mapper._props[key]
        strategy = prop._strategies[LazyLoader]

        return strategy._load_for_state(state, passive)


class ImmediateLoader(AbstractRelationshipLoader):
    def init_class_attribute(self, mapper):
        self.parent_property.\
                _get_strategy(LazyLoader).\
                init_class_attribute(mapper)

    def setup_query(self, context, entity, 
                        path, reduced_path, adapter, column_collection=None,
                        parentmapper=None, **kwargs):
        pass

    def create_row_processor(self, context, path, reduced_path, mapper, row, adapter):
        def execute(state, dict_, row):
            state.get_impl(self.key).get(state, dict_)

        return None, None, execute

class SubqueryLoader(AbstractRelationshipLoader):
    def init(self):
        super(SubqueryLoader, self).init()
        self.join_depth = self.parent_property.join_depth

    def init_class_attribute(self, mapper):
        self.parent_property.\
                _get_strategy(LazyLoader).\
                init_class_attribute(mapper)

    def setup_query(self, context, entity, 
                        path, reduced_path, adapter, column_collection=None,
                        parentmapper=None, **kwargs):

        if not context.query._enable_eagerloads:
            return

        path = path + (self.key, )
        reduced_path = reduced_path + (self.key, )

        # build up a path indicating the path from the leftmost
        # entity to the thing we're subquery loading.
        subq_path = context.attributes.get(('subquery_path', None), ())

        subq_path = subq_path + path

        # join-depth / recursion check
        if ("loaderstrategy", reduced_path) not in context.attributes:
            if self.join_depth:
                if len(path) / 2 > self.join_depth:
                    return
            else:
                if self.mapper.base_mapper in interfaces._reduce_path(subq_path):
                    return

        orig_query = context.attributes.get(
                                ("orig_query", SubqueryLoader), 
                                context.query)

        subq_mapper = mapperutil._class_to_mapper(subq_path[0])

        # determine attributes of the leftmost mapper
        if self.parent.isa(subq_mapper) and self.key==subq_path[1]:
            leftmost_mapper, leftmost_prop = \
                                    self.parent, self.parent_property
        else:
            leftmost_mapper, leftmost_prop = \
                                    subq_mapper, \
                                    subq_mapper._props[subq_path[1]]
        leftmost_cols, remote_cols = self._local_remote_columns(leftmost_prop)

        leftmost_attr = [
            leftmost_mapper._columntoproperty[c].class_attribute
            for c in leftmost_cols
        ]

        # reformat the original query
        # to look only for significant columns
        q = orig_query._clone()

        # TODO: why does polymporphic etc. require hardcoding 
        # into _adapt_col_list ?  Does query.add_columns(...) work
        # with polymorphic loading ?
        q._set_entities(q._adapt_col_list(leftmost_attr))

        # don't need ORDER BY if no limit/offset
        if q._limit is None and q._offset is None:
            q._order_by = None

        # the original query now becomes a subquery
        # which we'll join onto.
        embed_q = q.with_labels().subquery()
        left_alias = mapperutil.AliasedClass(leftmost_mapper, embed_q)

        # q becomes a new query.  basically doing a longhand
        # "from_self()".  (from_self() itself not quite industrial
        # strength enough for all contingencies...but very close)

        q = q.session.query(self.mapper)
        q._attributes = {
            ("orig_query", SubqueryLoader): orig_query,
            ('subquery_path', None) : subq_path
        }
        q = q._enable_single_crit(False)

        # figure out what's being joined.  a.k.a. the fun part
        to_join = [
                    (subq_path[i], subq_path[i+1]) 
                    for i in xrange(0, len(subq_path), 2)
                ]

        # determine the immediate parent class we are joining from,
        # which needs to be aliased.

        if len(to_join) < 2:
            # in the case of a one level eager load, this is the
            # leftmost "left_alias".
            parent_alias = left_alias
        elif subq_path[-2].isa(self.parent):
            # In the case of multiple levels, retrieve
            # it from subq_path[-2]. This is the same as self.parent 
            # in the vast majority of cases, and [ticket:2014] 
            # illustrates a case where sub_path[-2] is a subclass
            # of self.parent
            parent_alias = mapperutil.AliasedClass(subq_path[-2])
        else:
            # if of_type() were used leading to this relationship, 
            # self.parent is more specific than subq_path[-2]
            parent_alias = mapperutil.AliasedClass(self.parent)

        local_cols, remote_cols = \
                        self._local_remote_columns(self.parent_property)

        local_attr = [
            getattr(parent_alias, self.parent._columntoproperty[c].key)
            for c in local_cols
        ]
        q = q.order_by(*local_attr)
        q = q.add_columns(*local_attr)

        for i, (mapper, key) in enumerate(to_join):

            # we need to use query.join() as opposed to
            # orm.join() here because of the 
            # rich behavior it brings when dealing with 
            # "with_polymorphic" mappers.  "aliased"
            # and "from_joinpoint" take care of most of 
            # the chaining and aliasing for us.

            first = i == 0
            middle = i < len(to_join) - 1
            second_to_last = i == len(to_join) - 2

            if first:
                attr = getattr(left_alias, key)
            else:
                attr = key

            if second_to_last:
                q = q.join(parent_alias, attr, from_joinpoint=True)
            else:
                q = q.join(attr, aliased=middle, from_joinpoint=True)

        # propagate loader options etc. to the new query.
        # these will fire relative to subq_path.
        q = q._with_current_path(subq_path)
        q = q._conditional_options(*orig_query._with_options)

        if self.parent_property.order_by:
            # if there's an ORDER BY, alias it the same
            # way joinedloader does, but we have to pull out 
            # the "eagerjoin" from the query.
            # this really only picks up the "secondary" table
            # right now.
            eagerjoin = q._from_obj[0]
            eager_order_by = \
                            eagerjoin._target_adapter.\
                                copy_and_process(
                                    util.to_list(
                                        self.parent_property.order_by
                                    )
                                )
            q = q.order_by(*eager_order_by)

        # add new query to attributes to be picked up 
        # by create_row_processor
        context.attributes[('subquery', reduced_path)] = q

    def _local_remote_columns(self, prop):
        if prop.secondary is None:
            return zip(*prop.local_remote_pairs)
        else:
            return \
                [p[0] for p in prop.synchronize_pairs],\
                [
                    p[0] for p in prop.
                                        secondary_synchronize_pairs
                ]

    def create_row_processor(self, context, path, reduced_path, 
                                    mapper, row, adapter):
        if not self.parent.class_manager[self.key].impl.supports_population:
            raise sa_exc.InvalidRequestError(
                        "'%s' does not support object "
                        "population - eager loading cannot be applied." % 
                        self)

        reduced_path = reduced_path + (self.key,)

        if ('subquery', reduced_path) not in context.attributes:
            return None, None, None

        local_cols, remote_cols = self._local_remote_columns(self.parent_property)

        remote_attr = [
                        self.mapper._columntoproperty[c].key
                        for c in remote_cols]

        q = context.attributes[('subquery', reduced_path)]

        collections = dict(
                    (k, [v[0] for v in v]) 
                    for k, v in itertools.groupby(
                        q, 
                        lambda x:x[1:]
                    ))

        if adapter:
            local_cols = [adapter.columns[c] for c in local_cols]

        if self.uselist:
            def execute(state, dict_, row):
                collection = collections.get(
                    tuple([row[col] for col in local_cols]), 
                    ()
                )
                state.get_impl(self.key).\
                        set_committed_value(state, dict_, collection)
        else:
            def execute(state, dict_, row):
                collection = collections.get(
                    tuple([row[col] for col in local_cols]), 
                    (None,)
                )
                if len(collection) > 1:
                    util.warn(
                        "Multiple rows returned with "
                        "uselist=False for eagerly-loaded attribute '%s' "
                        % self)

                scalar = collection[0]
                state.get_impl(self.key).\
                        set_committed_value(state, dict_, scalar)

        return execute, None, None

log.class_logger(SubqueryLoader)

class EagerLoader(AbstractRelationshipLoader):
    """Strategize a relationship() that loads within the process 
    of the parent object being selected."""

    def init(self):
        super(EagerLoader, self).init()
        self.join_depth = self.parent_property.join_depth

    def init_class_attribute(self, mapper):
        self.parent_property.\
            _get_strategy(LazyLoader).init_class_attribute(mapper)

    def setup_query(self, context, entity, path, reduced_path, adapter, \
                                column_collection=None, parentmapper=None,
                                allow_innerjoin=True,
                                **kwargs):
        """Add a left outer join to the statement thats being constructed."""


        if not context.query._enable_eagerloads:
            return

        path = path + (self.key,)
        reduced_path = reduced_path + (self.key,)

        # check for user-defined eager alias
        if ("user_defined_eager_row_processor", reduced_path) in\
                context.attributes:
            clauses = context.attributes[
                                ("user_defined_eager_row_processor",
                                reduced_path)]

            adapter = entity._get_entity_clauses(context.query, context)
            if adapter and clauses:
                context.attributes[
                            ("user_defined_eager_row_processor",
                            reduced_path)] = clauses = clauses.wrap(adapter)
            elif adapter:
                context.attributes[
                            ("user_defined_eager_row_processor",
                            reduced_path)] = clauses = adapter

            add_to_collection = context.primary_columns

        else:
            # check for join_depth or basic recursion,
            # if the current path was not explicitly stated as 
            # a desired "loaderstrategy" (i.e. via query.options())
            if ("loaderstrategy", reduced_path) not in context.attributes:
                if self.join_depth:
                    if len(path) / 2 > self.join_depth:
                        return
                else:
                    if self.mapper.base_mapper in reduced_path:
                        return

            clauses = mapperutil.ORMAdapter(
                        mapperutil.AliasedClass(self.mapper), 
                        equivalents=self.mapper._equivalent_columns,
                        adapt_required=True)

            if self.parent_property.direction != interfaces.MANYTOONE:
                context.multi_row_eager_loaders = True

            innerjoin = allow_innerjoin and context.attributes.get(
                                ("eager_join_type", path), 
                                self.parent_property.innerjoin)
            if not innerjoin:
                # if this is an outer join, all eager joins from
                # here must also be outer joins
                allow_innerjoin = False

            context.create_eager_joins.append(
                (self._create_eager_join, context, 
                entity, path, adapter, 
                parentmapper, clauses, innerjoin)
            )

            add_to_collection = context.secondary_columns
            context.attributes[
                                ("eager_row_processor", reduced_path)
                              ] = clauses

        path += (self.mapper,)
        reduced_path += (self.mapper.base_mapper,)

        for value in self.mapper._polymorphic_properties:
            value.setup(
                context, 
                entity, 
                path, 
                reduced_path,
                clauses, 
                parentmapper=self.mapper, 
                column_collection=add_to_collection,
                allow_innerjoin=allow_innerjoin)

    def _create_eager_join(self, context, entity, 
                            path, adapter, parentmapper, 
                            clauses, innerjoin):

        if parentmapper is None:
            localparent = entity.mapper
        else:
            localparent = parentmapper

        # whether or not the Query will wrap the selectable in a subquery,
        # and then attach eager load joins to that (i.e., in the case of 
        # LIMIT/OFFSET etc.)
        should_nest_selectable = context.multi_row_eager_loaders and \
            context.query._should_nest_selectable

        entity_key = None
        if entity not in context.eager_joins and \
            not should_nest_selectable and \
            context.from_clause:
            index, clause = \
                sql_util.find_join_source(
                                context.from_clause, entity.selectable)
            if clause is not None:
                # join to an existing FROM clause on the query.
                # key it to its list index in the eager_joins dict.
                # Query._compile_context will adapt as needed and 
                # append to the FROM clause of the select().
                entity_key, default_towrap = index, clause

        if entity_key is None:
            entity_key, default_towrap = entity, entity.selectable

        towrap = context.eager_joins.setdefault(entity_key, default_towrap)

        join_to_left = False
        if adapter:
            if getattr(adapter, 'aliased_class', None):
                onclause = getattr(
                                adapter.aliased_class, self.key,
                                self.parent_property)
            else:
                onclause = getattr(
                                mapperutil.AliasedClass(
                                        self.parent, 
                                        adapter.selectable
                                ), 
                                self.key, self.parent_property
                            )

            if onclause is self.parent_property:
                # TODO: this is a temporary hack to 
                # account for polymorphic eager loads where
                # the eagerload is referencing via of_type().
                join_to_left = True
        else:
            onclause = self.parent_property

        context.eager_joins[entity_key] = eagerjoin = \
                                mapperutil.join(
                                            towrap, 
                                            clauses.aliased_class, 
                                            onclause, 
                                            join_to_left=join_to_left, 
                                            isouter=not innerjoin
                                            )

        # send a hint to the Query as to where it may "splice" this join
        eagerjoin.stop_on = entity.selectable

        if self.parent_property.secondary is None and \
                not parentmapper:
            # for parentclause that is the non-eager end of the join,
            # ensure all the parent cols in the primaryjoin are actually 
            # in the
            # columns clause (i.e. are not deferred), so that aliasing applied 
            # by the Query propagates those columns outward.
            # This has the effect 
            # of "undefering" those columns.
            for col in sql_util.find_columns(
                                self.parent_property.primaryjoin):
                if localparent.mapped_table.c.contains_column(col):
                    if adapter:
                        col = adapter.columns[col]
                    context.primary_columns.append(col)

        if self.parent_property.order_by:
            context.eager_order_by += \
                            eagerjoin._target_adapter.\
                                copy_and_process(
                                    util.to_list(
                                        self.parent_property.order_by
                                    )
                                )


    def _create_eager_adapter(self, context, row, adapter, path, reduced_path):
        if ("user_defined_eager_row_processor", reduced_path) in \
                                                    context.attributes:
            decorator = context.attributes[
                            ("user_defined_eager_row_processor",
                            reduced_path)]
            # user defined eagerloads are part of the "primary" 
            # portion of the load.
            # the adapters applied to the Query should be honored.
            if context.adapter and decorator:
                decorator = decorator.wrap(context.adapter)
            elif context.adapter:
                decorator = context.adapter
        elif ("eager_row_processor", reduced_path) in context.attributes:
            decorator = context.attributes[
                            ("eager_row_processor", reduced_path)]
        else:
            return False

        try:
            identity_key = self.mapper.identity_key_from_row(row, decorator)
            return decorator
        except KeyError, k:
            # no identity key - dont return a row 
            # processor, will cause a degrade to lazy
            return False

    def create_row_processor(self, context, path, reduced_path, mapper, row, adapter):
        if not self.parent.class_manager[self.key].impl.supports_population:
            raise sa_exc.InvalidRequestError(
                        "'%s' does not support object "
                        "population - eager loading cannot be applied." % 
                        self)

        our_path = path + (self.key,)
        our_reduced_path = reduced_path + (self.key,)

        eager_adapter = self._create_eager_adapter(
                                                context, 
                                                row, 
                                                adapter, our_path,
                                                our_reduced_path)

        if eager_adapter is not False:
            key = self.key
            _instance = self.mapper._instance_processor(
                                context, 
                                our_path + (self.mapper,), 
                                our_reduced_path + (self.mapper.base_mapper,),
                                eager_adapter)

            if not self.uselist:
                def new_execute(state, dict_, row):
                    # set a scalar object instance directly on the parent
                    # object, bypassing InstrumentedAttribute event handlers.
                    dict_[key] = _instance(row, None)

                def existing_execute(state, dict_, row):
                    # call _instance on the row, even though the object has
                    # been created, so that we further descend into properties
                    existing = _instance(row, None)
                    if existing is not None \
                        and key in dict_ \
                        and existing is not dict_[key]:
                        util.warn(
                            "Multiple rows returned with "
                            "uselist=False for eagerly-loaded attribute '%s' "
                            % self)
                return new_execute, existing_execute, None
            else:
                def new_execute(state, dict_, row):
                    collection = attributes.init_state_collection(
                                                    state, dict_, key)
                    result_list = util.UniqueAppender(collection,
                                                      'append_without_event')
                    context.attributes[(state, key)] = result_list
                    _instance(row, result_list)

                def existing_execute(state, dict_, row):
                    if (state, key) in context.attributes:
                        result_list = context.attributes[(state, key)]
                    else:
                        # appender_key can be absent from context.attributes
                        # with isnew=False when self-referential eager loading
                        # is used; the same instance may be present in two
                        # distinct sets of result columns
                        collection = attributes.init_state_collection(state,
                                        dict_, key)
                        result_list = util.UniqueAppender(
                                                collection,
                                                'append_without_event')
                        context.attributes[(state, key)] = result_list
                    _instance(row, result_list)
            return new_execute, existing_execute, None
        else:
            return self.parent_property.\
                            _get_strategy(LazyLoader).\
                            create_row_processor(
                                            context, path, 
                                            reduced_path,
                                            mapper, row, adapter)

log.class_logger(EagerLoader)

class EagerLazyOption(StrategizedOption):
    def __init__(self, key, lazy=True, chained=False,
                    propagate_to_loaders=True
                    ):
        super(EagerLazyOption, self).__init__(key)
        self.lazy = lazy
        self.chained = self.lazy in (False, 'joined', 'subquery') and chained
        self.propagate_to_loaders = propagate_to_loaders
        self.strategy_cls = factory(lazy)

    def get_strategy_class(self):
        return self.strategy_cls

def factory(identifier):
    if identifier is False or identifier == 'joined':
        return EagerLoader
    elif identifier is None or identifier == 'noload':
        return NoLoader
    elif identifier is False or identifier == 'select':
        return LazyLoader
    elif identifier == 'subquery':
        return SubqueryLoader
    elif identifier == 'immediate':
        return ImmediateLoader
    else:
        return LazyLoader



class EagerJoinOption(PropertyOption):

    def __init__(self, key, innerjoin, chained=False):
        super(EagerJoinOption, self).__init__(key)
        self.innerjoin = innerjoin
        self.chained = chained

    def process_query_property(self, query, paths, mappers):
        if self.chained:
            for path in paths:
                query._attributes[("eager_join_type", path)] = self.innerjoin
        else:
            query._attributes[("eager_join_type", paths[-1])] = self.innerjoin

class LoadEagerFromAliasOption(PropertyOption):

    def __init__(self, key, alias=None, chained=False):
        super(LoadEagerFromAliasOption, self).__init__(key)
        if alias is not None:
            if not isinstance(alias, basestring):
                m, alias, is_aliased_class = mapperutil._entity_info(alias)
        self.alias = alias
        self.chained = chained

    def process_query_property(self, query, paths, mappers):
        if self.chained:
            for path in paths[0:-1]:
                (root_mapper, propname) = path[-2:]
                prop = root_mapper._props[propname]
                adapter = query._polymorphic_adapters.get(prop.mapper, None)
                query._attributes.setdefault(
                            ("user_defined_eager_row_processor", 
                            interfaces._reduce_path(path)), adapter)

        if self.alias is not None:
            if isinstance(self.alias, basestring):
                (root_mapper, propname) = paths[-1][-2:]
                prop = root_mapper._props[propname]
                self.alias = prop.target.alias(self.alias)
            query._attributes[
                        ("user_defined_eager_row_processor", 
                        interfaces._reduce_path(paths[-1]))
                        ] = sql_util.ColumnAdapter(self.alias)
        else:
            (root_mapper, propname) = paths[-1][-2:]
            prop = root_mapper._props[propname]
            adapter = query._polymorphic_adapters.get(prop.mapper, None)
            query._attributes[
                        ("user_defined_eager_row_processor", 
                        interfaces._reduce_path(paths[-1]))] = adapter

def single_parent_validator(desc, prop):
    def _do_check(state, value, oldvalue, initiator):
        if value is not None:
            hasparent = initiator.hasparent(attributes.instance_state(value))
            if hasparent and oldvalue is not value: 
                raise sa_exc.InvalidRequestError(
                    "Instance %s is already associated with an instance "
                    "of %s via its %s attribute, and is only allowed a "
                    "single parent." % 
                    (mapperutil.instance_str(value), state.class_, prop)
                )
        return value

    def append(state, value, initiator):
        return _do_check(state, value, None, initiator)

    def set_(state, value, oldvalue, initiator):
        return _do_check(state, value, oldvalue, initiator)

    event.listen(desc, 'append', append, raw=True, retval=True, active_history=True)
    event.listen(desc, 'set', set_, raw=True, retval=True, active_history=True)

