title: How to add additional column to Django QuerySet
body: <p>I have a QuerySet with Books and I would like to add a <code>score</code> field to every Book result.</p>
  
  <pre><code>qs = Book.objects.all()
  </code></pre>
  
  <p>In raw SQL I would write:</p>
  
  <pre><code>SELECT
      *,
      (
          (SELECT COUNT(*) FROM votes WHERE value=1 AND book=b.id) - 
          (SELECT COUNT(*) FROM votes WHERE value=-1 AND book=b.id)
      ) AS score
  FROM
      Book b;
  </code></pre>
  
  <p>How can I achieve it in Django? I tried <code>annotate()</code>, but it seems it's not meant for this kind of stuff.</p>