docs: def lookups(self, request, model_admin):
          """
          Returns a list of tuples. The first element in each
          tuple is the coded value for the option that will
          appear in the URL query. The second element is the
          human-readable name for the option that will appear
          in the right sidebar.
          """
  
          User = get_user_model()
          output = []
          for i in models.Model.objects.values('user__pk').distinct():
              pk = i['user__pk']
  
              if pk is not None:
                  output.append([pk, User.objects.get(pk=pk).__str__])
  
          return output