django：如何停止创建新的主键保存（using =）

>>> a=Household.objects.filter(h_identifier=\'H122000-48\').using(\'db01\')
>>> a[0].pk
>>> u\'451465ea-0137-11e0-879a-70f1a16e0f80\'
>>> a[0].save(using=\'default\')
>>> b=Household.objects.filter(h_identifier=\'H122000-48\').using(\'default\')
>>> b[0].pk
>>> u\'7c2484fe-8641-11e0-b080-00188b4d6b0e\'

class FixedUUIDField(UUIDField):
def pre_save(self, model_instance, add):
    value = super(UUIDField, self).pre_save(model_instance, add)
    if self.auto and add and not value:
        value = unicode(self.create_uuid())
        setattr(model_instance, self.attname, value)
    return value