title: Django, use QuerySet.values() to created nested aggregation
body: <p>Using Django's aggregation tools, I can use </p>
  
  <pre><code>Author.objects.values('country', 'name').annotate(average_rating=Avg('book__rating'))
  </code></pre>
  
  <p>To get a list of <code>{name: XX, country: XX, average_rating: XX}</code> dictionaries. Using the plain django api and no raw SQL, is there a way of creating nested representation like</p>
  
  <pre><code>[{country: XX, results: [{name: XX, average_rating: XX}..], ..}]
  </code></pre>
  
  <p>?</p>