我有一个复杂的 sql 查询，不知道如何转化为 Rails 方式的查询，sql 如下

SELECT
 ID.LEVEL,
 t.*
FROM
(
 SELECT
 @id AS _id, ( SELECT @id := parent_id FROM product_categories WHERE id = @id),
 @l := @l + 1 AS LEVEL
 FROM
 product_categories, (SELECT @id := 100, @l := 0) as b
 WHERE
 @id > 0 ) as ID,
 product_categories as t
WHERE
 ID._id = t.id
ORDER BY `LEVEL`;

我尝试这样使用

ProductCategory.find_by_sql("xxx")

但是会报错。