title: get related object but not via foreign key
body: <p>I have the following models in my django app.</p>
  
  <pre><code>class EndUser(models.Model):
      end_user_id = models.CharField
  
  class EndUserField(models.Model):
      name = models.CharField
      value = models.CharField
      user = models.ForeignKey("EndUser", null=False, verbose_name=_('end user'))
  </code></pre>
  
  <p>An EndUser can have many EndUserFields.
  In some situations, if a certain EndUserField is queried, I would like to be able to fetch another field.</p>
  
  <p>I tried the following, but it does not work:</p>
  
  <pre><code>(
      EndUserField.objects
      .filter(name='x')
      .select_related('user')
      .prefetch_related(
          Prefetch(
              'user__enduserfield_set',
              queryset=(
                  EndUserField.objects
                  .filter(
                      name='y'
                  )
              ),
              to_attr='user_name'
          )
      )
  )
  </code></pre>