title: Ordering Django models by a method in the model
body: <p>So, say I have models like this:</p>
  
  <pre><code>class Foo(Model):
      name = CharField(max_length=200)
      def latest_comment(self):
          try:
              object = self.comment_set.latest()
              if object:
                  return object.when_posted.date()
          except:
              return ""
  
  class Comment(Model):
     when_posted = DateTimeField()
     text = TextField()
  </code></pre>
  
  <p>Then this is the modelAdmin:</p>
  
  <pre><code>class FooAdmin(ModelAdmin):
      list_display = ['name', 'latest_comment']
      ordering = ['latest_comment']
  
  admin.site.register(Foo, FooAdmin)
  </code></pre>
  
  <p>It throws an error when I go to the admin site saying that 'latest_comment' was not found in app.Foo. Having it in list_display works fine. So, my question is: Is there a way to order models in list_display by methods of the model? And if so, how?</p>