title: Django annotate Avg of foreign model
body: <p>Two models, article and review, relationship is one to many (one article has many reviews). Some articles don't have any review.</p>
  <p>I want to order articles by review ratings, therefore I use the annotate with AVG:</p>
  <pre><code>ArticleQueryset.annotate(rating=models.Avg('reviews__rating')).order_by('-rating')
  </code></pre>
  <p>The issue is that the articles without reviews the rating value is False and somehow that comes before the maximum rating. The result is that the first results don't have any rating, then the highest rated articles show up.</p>