Add queryset aggregates to allowed fields in OrderingFilter

rest_framework/filters.py

@@ -124,6 +124,7 @@ class OrderingFilter(BaseFilterBackend):
 
     def remove_invalid_fields(self, queryset, ordering):
         field_names = [field.name for field in queryset.model._meta.fields]
+        field_names += queryset.query.aggregates.keys()
         return [term for term in ordering if term.lstrip('-') in field_names]
 
     def filter_queryset(self, request, queryset, view):

rest_framework/tests/test_filters.py

@@ -363,6 +363,12 @@ class OrdringFilterModel(models.Model):
     text = models.CharField(max_length=100)
 
 
+class OrderingFilterRelatedModel(models.Model):
+    related_object = models.ForeignKey(OrdringFilterModel,
+                                       related_name="relateds")
+
+
+
 class OrderingFilterTests(TestCase):
     def setUp(self):
         # Sequence of title/text is:
@@ -472,3 +478,36 @@ class OrderingFilterTests(TestCase):
                 {'id': 1, 'title': 'zyx', 'text': 'abc'},
             ]
         )
+
+    def test_ordering_by_aggregate_field(self):
+        # create some related models to aggregate order by
+        num_objs = [2, 5, 3]
+        for obj, num_relateds in zip(OrdringFilterModel.objects.all(),
+                                     num_objs):
+            for _ in range(num_relateds):
+                new_related = OrderingFilterRelatedModel(
+                    related_object=obj
+                )
+                new_related.save()
+
+        class OrderingListView(generics.ListAPIView):
+            model = OrdringFilterModel
+            filter_backends = (filters.OrderingFilter,)
+            ordering = 'title'
+            queryset = OrdringFilterModel.objects.all().annotate(
+                models.Count("relateds"))
+
+        view = OrderingListView.as_view()
+        request = factory.get('?ordering=relateds__count')
+        response = view(request)
+        self.assertEqual(
+            response.data,
+            [
+                {'id': 1, 'title': 'zyx', 'text': 'abc'},
+                {'id': 3, 'title': 'xwv', 'text': 'cde'},
+                {'id': 2, 'title': 'yxw', 'text': 'bcd'},
+            ]
+        )
+
+
+