title: How can I set two primary key fields for my models in Django?
body: <p>I have a model like this:</p>
  <pre><code>class Hop(models.Model):
      migration = models.ForeignKey('Migration')
      host = models.ForeignKey(User, related_name='host_set')
  </code></pre>
  <p>How can I have the primary key be the combination of <code>migration</code> and <code>host</code>?</p>