title: Django: Ordering Queryset with a specific filtered field
body: <p>I need to sort the items queryset, introducing the obsolescence of content. (Content is ManytoMany related with Item class).
  Is it possible to filter the contents by pub_date within an annotation?</p>
  
  <p>My code works here for every content publication date. I need the annotate function to work for "pub_date > '2012-10-10'" (for example).</p>
  
  <pre><code>self.queryset = Item.objects.all()    
  self.queryset = self.queryset.annotate(
                  Count("content")).order_by("-content__count")
  </code></pre>
  
  <p>I tried to filter with the extra() function, which doesn't work:</p>
  
  <pre><code>self.queryset = self.queryset.extra(select={
  'content': "pub_date &gt; '2012-10-10'"}).annotate(
  Count("content")).order_by("-content__count")
  </code></pre>