title: Using a widget for aggregate value in Django Admin
body: <p>In my ModelAdmin, I have calculate two aggregate values and want to display those with a widget.</p>
  
  <pre><code>def get_queryset(self, request):
          qs = super(QuizAdmin, self).get_queryset(request).annotate(
              avg_difficulty=Avg('ratings__difficulty'),
              avg_rating=Avg('ratings__rating'),
          )
          return qs.filter(author__agents=request.user)
  </code></pre>
  
  <p>At the moment I'm doing this</p>
  
  <pre><code>def avg_difficulty(self, obj):
          return StarRatingWidget().render(attrs={'id': 'id_avg_difficulty'},name="avg_difficulty", value=obj.avg_difficulty, readonly=True)
  </code></pre>
  
  <p>Is there a simpler and more flexible way to use a widget for an aggregate value? Registering in the ModelForm does not seem to work.</p>