docs: def user_order_by(self, field):
      """
      Queryset method ordering objects by user ordering field.
      """
      # Get ordering model.
      model_label = order.utils.resolve_labels('.'.join(\
              [self.model._meta.app_label, self.model._meta.object_name]))
      orderitem_set = getattr(self.model, \
              order.utils.resolve_order_item_related_set_name(model_label))
      order_model = orderitem_set.related.model
  
      # Resolve ordering model table name.
      db_table = order_model._meta.db_table
  
      # Add ordering field as extra queryset fields.
      pk_name = self.model._meta.pk.attname
  
      # If we have a descending query remove '-' from field name when quering.
      sanitized_field = field.lstrip('-')
  
      extra_select = {
          sanitized_field: '(SELECT %s from %s WHERE item_id=%s.%s)' % \
                  (sanitized_field, db_table, self.model._meta.db_table, pk_name)
      }
  
      # Use original field name when ordering to allow for descending.
      return self.extra(select=extra_select).all().order_by(field)