ARel中的案例陈述 [英] Case Statements in ARel

问题描述

我正在尝试使用ARel清理用MySQL案例语句编写的ActiveRecord代码.我知道Arel可以处理案例陈述，但不确定如何使用它.

I'm trying to clean up an ActiveRecord code written with MySQL case statements using ARel. I know Arel is able to handle case statements, but not sure how to use it.

所以我有一个带有数量和user_id(外键)列的订单表.我正在动态计算订单价格，因为基于订购数量存在不同的定价方案.这是AR代码的样子:

So I have an orders table with quantity and user_id(foreign key) columns. I'm dynamically computing the price of orders as there are different pricing schemes based on the quantity ordered. This is what the AR code looks like:

def orders_with_price
  <<-SQL
   orders.*, 
   SUM(CASE orders.quantity
   WHEN 2 THEN 500 * orders.quantity   
   WHEN 5 THEN 450 * orders.quantity
   WHEN 10 THEN 350 * orders.quantity
   END) AS total_price
  SQL
end


Order.
select(orders_with_price).
group("orders.user_id").
having("total_price > ?", minimum_price).
order("total_price")

推荐答案

我认为从Arel v7.x引入 Arel :: Nodes :: Case (可用于case语句).您可以将查询改写为:

I think from Arel v7.x introduced Arel::Nodes::Case which can be used for case statements. You can re-write this query as:

def case_statements
  orders = Order.arel_table
  Arel::Nodes::Case.
  new(orders[:quantity]).
  when(2).then(orders[:quantity] * 500).
  when(5).then(orders[:quantity] * 450).
  when(10).then(orders[:quantity] * 350)
end
orders = Order.arel_table

Order.
select(Arel.star).
select(Arel::Nodes::Sum.new(case_statements).as("total_price")).
group(orders[:user_id]).
having("total_price > ?", minimum_price).
order("total_price")

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