title: prefetch_related for multiple Levels
body: <p>If my Models look like:</p>
  
  <pre><code>class Publisher(models.Model):
      pass
  
  class Book(models.Model):
      publisher = models.ForeignKey(Publisher)
  
  class Page(models.Model):
      book = models.ForeignKey(Book)
  </code></pre>
  
  <p>and I would like to get the queryset for <code>Publisher</code> I do <code>Publisher.object.all()</code>.
  If then want to make sure to prefetch I can do:</p>
  
  <pre><code>Publisher.objects.all().prefetch_related('book_set')`
  </code></pre>
  
  <p>My questions are:</p>
  
  <ol>
  <li>Is there a way to do this prefetching using <code>select_related</code> or
  must I use <code>prefetch_related</code>?</li>
  <li>Is there a way to prefetch the
  <code>page_set</code>? This does not work:</li>
  </ol>
  
  <p><code>Publisher.objects.all().prefetch_related('book_set', 'book_set_page_set')</code></p>