docs: def zadd(self, *args, **kwargs):
          """
          For each score/value given as paramter, do a "zadd" call with
          score/self.instance as parameter call for each value. Values must be
          primary keys of the related model.
          """
          if 'values_callback' not in kwargs:
              kwargs['values_callback'] = self._to_fields
          pieces = fields.SortedSetField.coerce_zadd_args(*args, **kwargs)
          for (score, related_field) in zip(*[iter(pieces)] * 2):
              related_method = getattr(related_field, 'zadd')
              related_method(score, self.instance._pk, values_callback=None)