docs: def _to_fields(self, *values):
          """
          Take a list of values, which must be primary keys of the model linked
          to the related collection, and return a list of related fields.
          """
          result = []
          for related_instance in values:
              if not isinstance(related_instance, model.RedisModel):
                  related_instance = self.related_field._model(related_instance)
              result.append(getattr(related_instance, self.related_field.name))
          return result