在rails上正确实现Ruby中的递归方法

repeat_purchases_percent is a method in my Category model. All parentless categories are guarenteed to have a value for future_purchases_percent in the database. Subcategories may or may not have a future_purchases_percent. if they do, i want to use that value. If not, I want to use the parent's value and so on.repeat_purchases_percent is a method in my Cate