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from django.contrib.contenttypes.models import ContentType
def fetch_content_objects(tagged_items, select_related_for=None):
"""
Retrieves ``ContentType`` and content objects for the given list of
``TaggedItems``, grouping the retrieval of content objects by model
type to reduce the number of queries executed.
This results in ``number_of_content_types + 1`` queries rather than
the ``number_of_tagged_items * 2`` queries you'd get by iterating
over the list and accessing each item's ``object`` attribute.
A ``select_related_for`` argument can be used to specify a list of
of model names (corresponding to the ``model`` field of a
``ContentType``) for which ``select_related`` should be used when
retrieving model instances.
"""
if select_related_for is None: select_related_for = []
# Group content object pks by their content type pks
objects = {}
for item in tagged_items:
objects.setdefault(item.content_type_id, []).append(item.object_id)
# Retrieve content types and content objects in bulk
content_types = ContentType._default_manager.in_bulk(objects.keys())
for content_type_pk, object_pks in objects.iteritems():
model = content_types[content_type_pk].model_class()
if content_types[content_type_pk].model in select_related_for:
objects[content_type_pk] = model._default_manager.select_related().in_bulk(object_pks)
else:
objects[content_type_pk] = model._default_manager.in_bulk(object_pks)
# Set content types and content objects in the appropriate cache
# attributes, so accessing the 'content_type' and 'object'
# attributes on each tagged item won't result in further database
# hits.
for item in tagged_items:
item._object_cache = objects[item.content_type_id][item.object_id]
item._content_type_cache = content_types[item.content_type_id]
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