也许你不需要这个字段，但是有一个singleton指向一个主键。沿着这些线：

Maybe you don't need that field but have a singleton pointing to a main key. Something along these lines:

class Key(models.Model):
    owner = models.ForeignKey(User)

    @property
    def is_main(self):
        return hasattr(self, 'is_main')

    def set_main(self):
        # update MainKey instance here


class MainKey(models.Model):
    key = models.ForeignKey(Key, related_name='is_main')

    def save(self, *args, **kwargs):
        self.id = 1
        super(MainKey, self).save(*args, **kwargs)

    def delete(self, *args, **kwargs):
        pass