forum = get_object_or_404(Forum, slug=slug)
if not forum.category.can_access(request.user):
     return HttpResponseForbidden()

The problem is that two SQL queries occur here, one during the get_object_or_404(), and then another in the following if statement, when we access category, a foreign key on the forum object. It would sure be nice to somehow use a select_related() there to avoid the extra SQL query. I did some googling, and found a quick tip on one of the This Week in Django podcast pages. And yes, the documentation confirmed that get_object_or_404() can now take as a first argument either a model, a manager, or a queryset!

So now you can keep using the handy get_object_or_404() idiom, and reduce the number of queries with a slight bit of refactoring:

forum = get_object_or_404(Forum.objects.select_related(), slug=slug)
if not forum.category.can_access(request.user):
     return HttpResponseForbidden()

Very cool!