Generic Foreign Keys can't be used as lhs in lookups for historical
reasons. Django 1.6 gave a FieldDoesNotExist exception when using GFKs
as lhs in lookups, but due to regression caused by lookup refactor patch
(20bab2cf9d02a5c6477d8aac066a635986e0d3f3) the exception type was
changed to AttributeError.
It might be a good idea to add support for gfk__exact and gfk__in lookups
later on.
Thanks to glicerinu@gmail.com for the report. The code in this commit
was written by Tim Graham.
Backport of efe87d3e48 from master