          # Similar to what Django does for cyclically related models.
          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
  
      def serialize_key(self, key):
          """
          Keys serialize to their string value,
          unless they exist in the `rename` dict.
 
</s> remove             class OnTheFlySerializer(self.__class__):
                fields = info
                parent = getattr(self, 'parent') or self
 </s> add 
        # Otherwise use `related_serializer` or fall back to
        # `OnTheFlySerializer` preserve custom serialization methods.
        return getattr(self, 'related_serializer') or OnTheFlySerializer
          # Similar to what Django does for cyclically related models.
          elif isinstance(info, str) and info in _serializers:
              return _serializers[info]
  
        # Otherwise use `related_serializer` or fall back to
        # `OnTheFlySerializer` preserve custom serialization methods.
        return getattr(self, 'related_serializer') or OnTheFlySerializer
        # 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):
          """
          Keys serialize to their string value,
          unless they exist in the `rename` dict.
keep keep keep keep replace replace keep keep keep keep keep
 Don't ever return the normal serializer again. 
djangorestframework/serializer.py