title: How to define a Many-To-Many relationship to another app?
body: <p>I have two apps:
  Ads
  Domains</p>
  
  <p>An Ad can be associated via Many-To-Many to a Domain. In my Ad model I have put:</p>
  
  <pre><code>domains = models.ManyToManyField(Domain)
  </code></pre>
  
  <p>How do I make it so it uses the Domains app and not the Ads app?</p>
  
  <p>Thanks</p>