Quote order_column_name 

when using reserved words as `order_column_name` or as `left_column_name` the unquoted name will raise errors

    ActiveRecord::StatementInvalid: Mysql2::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1: SELECT `categories`.* FROM `categories`  WHERE `categories`.`parent_id` = 7 ORDER BY left