Loanding pins does no N+1 queries anymore

This saves another three queries per pin, bringing to total query count
down to 8.

See issue #85 .

pinry/core/api.py

@@ -1,3 +1,4 @@
+from django.core.exceptions import ObjectDoesNotExist
 from tastypie import fields
 from tastypie.authorization import DjangoAuthorization
 from tastypie.constants import ALL, ALL_WITH_RELATIONS
@@ -59,7 +60,10 @@ class UserResource(ModelResource):
 
 def filter_generator_for(size):
     def wrapped_func(bundle, **kwargs):
-        return bundle.obj.get_by_size(size)
+        for thumbnail in bundle.obj._prefetched_objects_cache['thumbnail']:
+            if thumbnail.size == size:
+                return thumbnail
+        raise DoesNotExist()
     return wrapped_func
 
 
@@ -136,7 +140,7 @@ class PinResource(ModelResource):
             'submitter': ALL_WITH_RELATIONS
         }
         queryset = Pin.objects.all().select_related('submitter'). \
-            prefetch_related('image', 'tags')
+            prefetch_related('image__thumbnail_set', 'tags')
         resource_name = 'pin'
         include_resource_uri = False
         always_return_data = True