title: django prefetch within a prefetch - must be a Model, Manager, or QuerySet, not 'QuerySet'
body: <p>Im trying to do a prefetch within a prefetch in django but am getting the below error:</p>
  
  <pre><code>First argument to get_object_or_404() must be a Model, Manager, or QuerySet, not 'QuerySet'.
  </code></pre>
  
  <p>this is the query:</p>
  
  <pre><code>site = get_object_or_404(SiteData.objects.prefetch_related(
                                  Prefetch(
                                      'sitesubnets_set',
                                      queryset=SiteSubnets.objects.filter(site_ip=True),
                                  ),
                                  Prefetch(
                                      'circuits_set',
                                      queryset=Circuits.objects.exclude(decommissioned=True).prefetch_related('servicecontacts_set'),
                                  )
                              ),
                          pk=site_id 
                          )
  </code></pre>
  
  <p>if I remove the below, the query works successfully, so I know its related to chaining the prefetches im just not sure why or how to rectify this?</p>
  
  <pre><code>.prefetch_related('servicecontacts_set')
  </code></pre>
  
  <p>EDIT:</p>
  
  <p>tried the below</p>
  
  <pre><code>site = get_object_or_404(SiteData.objects.prefetch_related(
                                  Prefetch(
                                      'sitesubnets_set',
                                      queryset=SiteSubnets.objects.filter(site_ip=True),
                                  ),
                                  Prefetch(
                                      'circuits_set',
                                      queryset=Circuits.objects.exclude(decommissioned=True),
                                  )
                              ),
                          pk=site_id 
                          )         
  site.circuits_set.prefetch_related('servicecontacts_set')
  </code></pre>
  
  <p>error:</p>
  
  <pre><code>AttributeError: Cannot find 'servicecontacts_set' on Circuits object, 'servicecontacts_set' is an invalid parameter to prefetch_related()
  </code></pre>
  
  <p>service contacts is a model linked to circuits, service contacts is not linked to site data if that helps?</p>
  
  <pre><code>&gt;&gt;&gt; vars(site.circuits_set.all()[0])
  {'_state': &lt;django.db.models.base.ModelState object at 0x7fa05dcf2978&gt;, 'id': 5, 'site_data_id': 7, 'order_no': 'N/A', 'expected_install_date': datetime.date(2016, 10, 19), 'install_date': datetime.date(2016, 5, 26), 'service_contacts_id': 10, 'decommissioned': False, '_site_data_cache': &lt;SiteData: London&gt;}
  &gt;&gt;&gt;
  </code></pre>