Returning Django Comments for Tastypie Resource

My Django website has a photo model representing photos on the system, and I use Django.contrib.commentsit so that users can comment on them. This all works fine, but I would like to expand my Tastypie API to allow access to comments for mine PhotoResourceusing a URL, for example /api/v1/photo/1/comments, where 1 is the photo id. I can make the URL work fine, but no matter what filtering I do, I always return the full set of comments, not just the set for the supplied photo. I have included a cut-out selection of my current API code below:

class CommentResource(ModelResource):
    user = fields.ForeignKey(UserResource, 'user')
    class Meta:
       queryset = Comment.objects.all()
            filtering = {
                'user': ALL_WITH_RELATIONS,
            }

class PhotoResource(ModelResource):
    user = fields.ForeignKey(UserResource, 'user')  
    class Meta:
        queryset = Photo.objects.all()
        filtering = {
            'id': 'exact',
            'user': ALL_WITH_RELATIONS
        }

    def prepend_urls(self):
        return [url(r"^(?P<resource_name>%s)/(?P<pk>\w[\w/-]*)/comments%s$" % (self._meta.resource_name, trailing_slash()), self.wrap_view('get_comments'), name="api_get_comments"),
        ]

    def get_comments(self, request, **kwargs):
        try:
            obj = self.cached_obj_get(request=request, **self.remove_api_resource_names(kwargs))
        except ObjectDoesNotExist:
            return HttpGone()
        except MultipleObjectsReturned:
            return HttpMultipleChoices("More than one resource is found at this URI.")
        comment_resource = CommentResource()
        return comment_resource.get_list(request, object_pk=obj.id, content_type=ContentType.objects.get_for_model(Photo))

, . , - contrib.comments, ContentTypes, , , , , , Tastypie . , . , - :

ctype = ContentType.objects.get_for_model(obj)
comment_resource = CommentResource()
return comment_resource.get_list(request, object_pk=obj.pk, content_type_id=ctype.id)

.

- , ( )?