title: Create Django Table displaying information about users
body: <p>I am currently using django 1.8 and I'd like to create a more intelligent way to display information about users.  Say I have something like this:</p>
  
  <pre><code>class User(models.Model):
      name = models.CharField()
  
  class Invitation(models.Model):
      inviter = models.ForeignKey(User)
      invitee = models.ForeignKey(User)
  </code></pre>
  
  <p>I want to create a field that is the unique number of user's an inviter has invited.  I could see how this could be done with something like <code>set("SELECT invitee FROM INVITATIONS WHERE inviter = 'my-user';")</code>, but if I want this displayed in the admin panel, is there a simple way to present this?</p>
  
  <p>Also, I would want this done for every user, so it feels like there is a simple way to make a field generated for every user in the table.</p>