我遇到一种情况，我想使用Meta选项unique_together来强制执行某条规则，这是中介模型：

class UserProfileExtension(models.Model):
    extension = models.ForeignKey(Extension, unique=False)
    userprofile = models.ForeignKey(UserProfile, unique=False)
    user = models.ForeignKey(User, unique=False)

    class Meta:
        unique_together = (("userprofile", "extension"),
                           ("user", "extension"),
                           # How can I enforce UserProfile's Client 
                           # and Extension to be unique? This obviously
                           # doesn't work, but is this idea possible without
                           # creating another FK in my intermediary model 
                           ("userprofile__client", "extension"))

这是UserProfile：

class UserProfile(models.Model):
    user = models.ForeignKey(User, unique=True)
    client = models.ForeignKey(Client)

谢谢。