title: django prefetch_related id only
body: <p>I'm trying to optimise my queries but prefetch_related insists on joining the tables and selecting all the fields even though I only need the list of ids from the relations table.</p>
  
  <p><img src="https://i.stack.imgur.com/4Sg50.png" alt="queries"></p>
  
  <p>You can ignore the 4th query. It's not related to the question.</p>
  
  <p>Related Code:</p>
  
  <pre><code>class Contact(models.Model):
      ...
      Groups = models.ManyToManyField(ContactGroup, related_name='contacts')
      ...
  
  queryset = Contact.objects.all().prefetch_related('Groups')
  </code></pre>