More robust default behavior on OrderingFilter (#4156)

rest_framework/filters.py

@@ -222,24 +222,40 @@ class OrderingFilter(BaseFilterBackend):
             return (ordering,)
         return ordering
 
-    def get_valid_fields(self, queryset, view):
-        valid_fields = getattr(view, 'ordering_fields', self.ordering_fields)
-
-        if valid_fields is None:
-            # Default to allowing filtering on serializer fields
+    def get_default_valid_fields(self, queryset, view):
+        # If `ordering_fields` is not specified, then we determine a default
+        # based on the serializer class, if one exists on the view.
+        if hasattr(view, 'get_serializer_class'):
             try:
                 serializer_class = view.get_serializer_class()
-            except AssertionError:  # raised if no serializer_class was found
-                msg = ("Cannot use %s on a view which does not have either a "
+            except AssertionError:
+                # Raised by the default implementation if
+                # no serializer_class was found
+                serializer_class = None
+        else:
+            serializer_class = getattr(view, 'serializer_class', None)
+
+        if serializer_class is None:
+            msg = (
+                "Cannot use %s on a view which does not have either a "
                 "'serializer_class', an overriding 'get_serializer_class' "
-                       "or 'ordering_fields' attribute.")
+                "or 'ordering_fields' attribute."
+            )
             raise ImproperlyConfigured(msg % self.__class__.__name__)
 
-            valid_fields = [
+        return [
             (field.source or field_name, field.label)
             for field_name, field in serializer_class().fields.items()
             if not getattr(field, 'write_only', False) and not field.source == '*'
         ]
+
+    def get_valid_fields(self, queryset, view):
+        valid_fields = getattr(view, 'ordering_fields', self.ordering_fields)
+
+        if valid_fields is None:
+            # Default to allowing filtering on serializer fields
+            return self.get_default_valid_fields(queryset, view)
+
         elif valid_fields == '__all__':
             # View explicitly allows filtering on any model field
             valid_fields = [