Don't ever return the normal serializer again.

djangorestframework/serializer.py

@@ -135,10 +135,12 @@ class Serializer(object):
         # If an element in `fields` is a 2-tuple of (str, tuple)
         # then the second element of the tuple is the fields to
         # set on the related serializer
-        if isinstance(info, (list, tuple)):
+
         class OnTheFlySerializer(self.__class__):
             fields = info
             parent = getattr(self, 'parent') or self
+
+        if isinstance(info, (list, tuple)):
             return OnTheFlySerializer
 
         # If an element in `fields` is a 2-tuple of (str, Serializer)
@@ -156,8 +158,9 @@ class Serializer(object):
         elif isinstance(info, str) and info in _serializers:
             return _serializers[info]
 
-        # Otherwise use `related_serializer` or fall back to `Serializer`
-        return getattr(self, 'related_serializer') or Serializer
+        # Otherwise use `related_serializer` or fall back to
+        # `OnTheFlySerializer` preserve custom serialization methods.
+        return getattr(self, 'related_serializer') or OnTheFlySerializer
 
     def serialize_key(self, key):
         """