如何访问ActiveRecord中SQL SUM函数的结果

Question

下面的SQL查询在Postgres中产生一个精确的计算:

select sum(invoice_items.unit_price * discounts.percentage/100) as discount from invoice_items join items on items.id = invoice_items.item_id join merchants on merchants.id = items.merchant_id join discounts on discounts.merchant_id = merchants.id where invoice_items.quantity >= discounts.threshold and invoice_items.invoice_id = 513;

对应的ActiveRecord正在生成成功的SQL语句:

invoice_items.select('sum(invoice_items.unit_price * discounts.percentage/100)')
.joins(:discounts)
.where('invoice_items.quantity >= discounts.threshold')

AR返回一个ActiveRecord_AssociationRelation…但是我不能让AR输出实际的SUM。

问题:如何更新AR表达式以返回SUM函数的实际值?

我试过了:

1. 将SUM函数混叠，然后在生成的AssocationRelation上调用它
2. 通过AssociationRelation迭代调用别名(方法不存在)
3. 删除SUM函数，添加别名，然后迭代结果，但我需要返回和....

Answer 1

可能问这个问题太早了....我已经解决了这个问题。

我更新ActiveRecord查询如下:

invoice_items.select('invoice_items.unit_price * discounts.percentage/100 as calculated_discount')
.joins(:discounts)
.where('invoice_items.quantity >= discounts.threshold')

然后在视图中调用

@invoice.discount.sum(&:calculated_discount)