[Django]複数カラムをまとめて一意(unique)にする

class User
    name = models.CharField(max_length=255)

class Position
    name = models.CharField(max_length=255)

class UserPosition(models.Model):
    user = models.ForeignKey(User)
    poistion = models.ForeignKey(Position)

    class Meta:
        unique_together (('user', 'poistion'),)

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