code: def user_default_serializer(self, obj):
          
          if not obj:
              return None
          self.user_default_add_related_pks(obj)
          return dict((
              ('id', obj.id),
              ('username', obj.username),
              self.field_to_json('DateTime', 'date_joined', obj.date_joined),
              self.field_to_json(
                  'PKList', 'votes', model=Choice, pks=obj._votes_pks),
          ))
docs: Convert a User to a cached instance representation.