title: Django Admin - sorting by custom method
body: <p>I know that it can to be done with annotate if you have some aggregation, but I was not able to implement it based on custom method. Bellow is the code for my model, and i want to be able to sort the results by _is_registered.</p>
  
  <pre><code>class Subscribers(models.Model):
      email = models.EmailField(_('E-mail'))
      want_newsletter = models.BooleanField(default = False)
  
      def get_user(self):
          user = User.objects.filter(email= self.email)
          if user.count()&gt;0:
              return user[0]
          return None
  
      def _is_registered(self):
          user = self.get_user()
          if user:
              return _('Yes')
          return _('No')
  </code></pre>