adds get_ordering method to GenericAPIView

following the pattern of e.g. `django.contrib.admin.ModelAdmin.get_fieldsets`, this PR adds an overrideable accessor to get `viewset.ordering` in order to enable dynamic default orderings. An example of where this is useful is in the case of a SearchFilter which returns results ranked based on similarity, in which case the default ordering should be ignored/bypassed.

rest_framework/filters.py

@@ -242,10 +242,10 @@ def get_ordering(self, request, queryset, view):
                 return ordering
 
         # No ordering was included, or all the ordering fields were invalid
-        return self.get_default_ordering(view)
+        return self.get_default_ordering(request, queryset, view)
 
-    def get_default_ordering(self, view):
-        ordering = getattr(view, 'ordering', None)
+    def get_default_ordering(self, request, queryset, view):
+        ordering = view.get_ordering(request, queryset)
         if isinstance(ordering, str):
             return (ordering,)
         return ordering

rest_framework/generics.py

@@ -104,6 +104,15 @@ def get_object(self):
 
         return obj
 
+    def get_ordering(self, request, queryset):
+        """
+        Returns the default ordering defined on the view.
+
+        You may want to override this if you wish the default ordering to be dependent
+        on the request.
+        """
+        return getattr(self, "ordering", None)
+
     def get_serializer(self, *args, **kwargs):
         """
         Return the serializer instance that should be used for validating and