code: def prepare_sort_key(self):
          '''
          Triggered by view_function._sort_converters when our sort key should be created.
          This can't be called in the constructor because Django models might not be ready yet.
          '''
          if isinstance(self.convert_type, str):
              try:
                  app_name, model_name = self.convert_type.split('.')
              except ValueError:
                  raise ImproperlyConfigured('"{}" is not a valid converter type. String-based converter types must be specified in "app.Model" format.'.format(self.convert_type))
              try:
                  self.convert_type = apps.get_model(app_name, model_name)
              except LookupError as e:
                  raise ImproperlyConfigured('"{}" is not a valid model name. {}'.format(self.convert_type, e))
          self.sort_key = ( -1 * len(inspect.getmro(self.convert_type)), -1 * self.source_order )