code: def choice_default_serializer(self, obj):
          
          if not obj:
              return None
          self.choice_default_add_related_pks(obj)
          return dict((
              ('id', obj.id),
              ('choice_text', obj.choice_text),
              self.field_to_json(
                  'PK', 'question', model=Question, pk=obj.question_id),
              self.field_to_json(
                  'PKList', 'voters', model=User, pks=obj._voter_pks)
          ))
docs: Convert a Choice to a cached instance representation.