title: Searching by Username field in Django admin
body: <p>I would like to know how to search using the details in the username column of my Django admin without directly knowing the username.</p>
  
  <p>I have objects in my admin and I want to be able to seach by the USERNAME_FIELD to access these objects about a given user.</p>
  
  <p>At first I tried:</p>
  
  <pre><code> search_fields = ('user__username', 'user__email',)
  </code></pre>
  
  <p>But that gave a FieldError: Cannot resolve keyword 'username' into field. </p>
  
  <p>I'm now trying something I found in the Django documentation but it's giving me an error which says 'too many values to unpack'.</p>
  
  <pre><code>def get_search_results(self, request, queryset, search_term):
       queryset, use_distinct = super(MyAdmin, self).get_search_results(request, queryset, search_term)
       queryset = self.model.objects.filter(get_user_model().USERNAME_FIELD)
       return queryset, use_distinct
  </code></pre>
  
  <p>The only solution I can think of is formatting a string which I'd rather not do since it's not very good code.</p>