Fixed #11039: documented that aggregation and generic relations don't mix. Thanks, psmith.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10781 bcc190cf-cafb-0310-a4f2-bffc1f526a37

1 files changed, 15 insertions, 0 deletions

diff --git a/docs/ref/contrib/contenttypes.txt b/docs/ref/contrib/contenttypes.txt
index 05862bc342..f814eccaab 100644
--- a/docs/ref/contrib/contenttypes.txt
+++ b/docs/ref/contrib/contenttypes.txt
@@ -339,6 +339,21 @@ pointing at it will be deleted as well. In the example above, this means that
 if a ``Bookmark`` object were deleted, any ``TaggedItem`` objects pointing at
 it would be deleted at the same time.
 
+Generic relations and aggregation
+---------------------------------
+
+:ref:`Django's database aggregation API <topics-db-aggregation`
+doesn't work with a
+:class:`~django.contrib.contenttypes.generic.GenericRelation`. For example, you
+might be tempted to try something like::
+
+    Bookmark.objects.aggregate(Count('tags')) 
+     
+This will not work correctly, however. The generic relation adds extra filters
+to the queryset to ensure the correct content type, but the ``aggregate`` method
+doesn't take them into account. For now, if you need aggregates on generic
+relations, you'll need to calculate them without using the aggregation API.
+
 Generic relations in forms and admin
 ------------------------------------