Use get_serializer_class in ordering filter

OrderingFilter backend checks whether a view specifies an
ordering fields or serializer_class attribute. Views can however
override get_serializer_class to determine the serializer_class
but the OrderingFilter doesn't catch this. This patch fixes that.

rest_framework/filters.py

@@ -228,9 +228,12 @@ class OrderingFilter(BaseFilterBackend):
         if valid_fields is None:
             # Default to allowing filtering on serializer fields
             serializer_class = getattr(view, 'serializer_class')
+            if serializer_class is None:
+                serializer_class = view.get_serializer_class()
                 if serializer_class is None:
                     msg = ("Cannot use %s on a view which does not have either a "
-                       "'serializer_class' or 'ordering_fields' attribute.")
+                           "'serializer_class', an overriding 'get_serializer_class' "
+                           "or 'ordering_fields' attribute.")
                     raise ImproperlyConfigured(msg % self.__class__.__name__)
             valid_fields = [
                 (field.source or field_name, field.label)

tests/test_filters.py

@@ -754,6 +754,28 @@ class OrderingFilterTests(TestCase):
 
         self.assertContains(response, 'verbose title')
 
+    def test_ordering_with_overridden_get_serializer_class(self):
+        class OrderingListView(generics.ListAPIView):
+            queryset = OrderingFilterModel.objects.all()
+            filter_backends = (filters.OrderingFilter,)
+            ordering = ('title',)
+            # note: no ordering_fields and serializer_class speficied
+
+            def get_serializer_class(self):
+                return OrderingFilterSerializer
+
+        view = OrderingListView.as_view()
+        request = factory.get('/', {'ordering': 'text'})
+        response = view(request)
+        self.assertEqual(
+            response.data,
+            [
+                {'id': 1, 'title': 'zyx', 'text': 'abc'},
+                {'id': 2, 'title': 'yxw', 'text': 'bcd'},
+                {'id': 3, 'title': 'xwv', 'text': 'cde'},
+            ]
+        )
+
 
 class SensitiveOrderingFilterModel(models.Model):
     username = models.CharField(max_length=20)