title: How to tell Django model that only one or other field must be not empty?
body: <p>With Django models.Model class like that:</p>
  <pre><code>class OrderLign(models.Model):
      Order = models.ForeignKey(Order,on_delete=models.CASCADE)
      Item = models.ForeignKey(Item, on_delete=models.RESTRICT, blank=True, default=0, null=True)
      Pack = models.ForeignKey(Pack,on_delete=models.RESTRICT, blank=True, default=0,
                               null=True)
  </code></pre>
  <p>I need that is possible to save OrderLign only if Item OR Pack is set, not both, and not neither.</p>