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.

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)