Django GenericForeignKey lookup for a given model

Question

I use a voting app (django-ratings if that makes any difference) which uses django's GenericForeignKey, has a ForeignKey to User, and several other fields like date of latest change.

I'd like to get all the objects of one content type, that a single user voted for ordered by date of latest change. As far as I understand - all the info can be found in a single table (except the content_type which can be prefetched/cached). Unfortunately django still makes an extra query each time I request a content_object.

So the question is - how do I get all the votes on a given model, by a given user, with related objects and given ordering with minimum database hits?

Edit: Right now I'm using 2 queries - first selecting all the votes, getting all the objects I need, filtering by .filter(pk__in=obj_ids) and finally populating them to votes objects. But it seems that a reverse generic relation can help solve the problem

Answer 1

Have you checked out select_related()? That may help.

Returns a QuerySet that will automatically "follow" foreign-key relationships, selecting that additional related-object data when it executes its query. This is a performance booster which results in (sometimes much) larger queries but means later use of foreign-key relationships won't require database queries.

https://docs.djangoproject.com/en/dev/ref/models/querysets/#select-related

Answer 2

Well right now we're using prefetch_related() from django 1.4 on a GenericRelation. It still uses 2 queries, but has a very intuitive interface.

Answer 3

From looking at the models.py of the django-ratings app, I think you would have to do user.votes.filter(content_type__model=Model._meta.module_name).order_by("date_changed") (assuming the model you want to filter by is Model) to get all the Vote objects. For the related objects, loop through the queryset getting content_object on each item. IMHO, this would result in the least DB queries.