id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
10870	Aggregates with joins ignore extra filters provided by setup_joins	fas	fas	"(In case the description that follows is not understandable, please ask me for a better explanation).

An example where extra filters in joins are used are reverse generic relations ([http://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/#reverse-generic-relations]).

{{{
class TaggedItem(models.Model):

    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = generic.GenericForeignKey('content_type', 'object_id')


class Bookmark(models.Model):
    url = models.URLField()
    tags = generic.GenericRelation(TaggedItem)
}}}

By querying {{{ Bookmark.objects.filter(tags=some_tag) }}} the generic relation adds extra filters to ensure the correct content type. Those extra filters however are ignored when using aggregates, which leads to wrong results.

For example, the following query is not correct because it does not check the content_type.
{{{
Bookmark.objects.aggregate(Count('tags'))
}}}
What this aggregate actually calculates per bookmark is not the amount of tags for this bookmark but the amount of tags for any model referenced by TaggedItem with the same object id.

The error is isolated in db/models/sql/query.py in the add_aggregate method:

{{{
            field, source, opts, join_list, last, _ = self.setup_joins(
                field_list, opts, self.get_initial_alias(), False)

            # Process the join chain to see if it can be trimmed                                                                                             
            col, _, join_list = self.trim_joins(source, join_list, last, False)
}}}

The last dummy element _ is normally named 'extra' in other uses of setup_joins and contains the extra filters needed to make it work. Adding {{{ self.add_filter(*extra) }}} after setup_joins fixes this problem.

"	Bug	closed	Database layer (models, ORM)	dev	Normal	fixed	orm, aggregation, join, contenttypes, filter	mbencun@… jdunck@… michaelg@… Robin Noah Kantrowitz Carl Meyer Chris Chambers joeri@… anssi.kaariainen@… ris Alan Justino da Silva daevaorn@… fhahn Hervé Cauwelier	Accepted	1	0	0	0	0	0