title: Why is the Prefetch object being ignored in prefetch_related?
body: <p>Using Django 1.7:</p>
  
  <p>My Queryset looks like this:</p>
  
  <pre><code>    return super(EmployeeViewSet, self).get_queryset()\
             .filter(status__deleted_flag=False, **filter_kwargs)\
             .prefetch_related('phone_number_set', 'email_address_set', 'street_address_set',
                               Prefetch('file_set', EmployeeFile.objects.active().select_related('content_type')))\
             .order_by('last_name', 'first_name')
  </code></pre>
  
  <p>DjDT shows the prefetch query executed</p>
  
  <pre><code>SELECT ••• FROM "employees_employeefile" INNER JOIN "employees_employeefiletype" 
  ON ("employees_employeefile"."content_type_id" = "employees_employeefiletype"."id" )
  WHERE ("employees_employeefile"."deleted_at" IS NULL 
         AND "employees_employeefile"."owner_id" IN (53, 81, ...)) 
  ORDER BY "employees_employeefile"."created_at" DESC
  </code></pre>
  
  <p>Then DjDT shows each employee getting requeried for the same list of files</p>
  
  <pre><code>SELECT ••• FROM "employees_employeefile" 
  WHERE ("employees_employeefile"."owner_id" = 53 
         AND "employees_employeefile"."deleted_at" IS NULL) 
  ORDER BY "employees_employeefile"."created_at" DESC
  </code></pre>
  
  <p>They queries look the same to me, and the ModelManager method being used to do the select is the same one <code>(EmployeeFile.objects.active())</code> vs <code>employee.file_set.active()</code></p>
  
  <p>Also, I tried removing the <code>select_related('content_type')</code> just in case that was the problem</p>