code: def append(self, hcont, value, score = None):
          
          assert (score is None) != (self.field.sort_field is None)
          if score is None:
              score = getattr(value, self.field.sort_field.name)
          ContainerFieldWriter.append(self, hcont, value, score)
docs: If sort_field is specified, score must be None.
              If sort_field is not specified, score is mandatory.