带有嵌套模型和派生列值的ActiveRecord查询？ [英] ActiveRecord query with nested models and derived column values?

问题描述

我有以下关系：

class Order < ActiveRecord::Base
  has_many :item_selections, :dependent => :destroy
  has_many :inventory_items, :through => :item_selections
end
class InventoryItem < ActiveRecord::Base
  has_many :item_selections, :dependent => :destroy
  has_many :orders, :through => :item_selections
end
class ItemSelection < ActiveRecord::Base
  belongs_to :order
  belongs_to :inventory_item
end

<我试图在下面创建与此SQL查询等效的ActiveRecord，然后加载* total_weight *& * total_volume *列到实例变量中：

I am trying to create the ActiveRecord equivalent of this SQL query below and then load the sum of the *total_weight* & *total_volume* columns into an instance variable:

select t1.quantity, t2.volume, t2.weight, 
t2.volume * t1.quantity as total_volume,    
t1.quantity * t2.weight as total_weight
from orders t0
inner join item_selections t1 on t0.id = t1.order_id
inner join inventory_items t2 on t1.inventory_item_id = t2.id
where t0.id = <id_val>     

是否有使用ActiveRecord获取这些值的正确方法？

Any idea on the right way to get these values using ActiveRecord?

推荐答案

这应该起作用：

orders = Order.select('orders.*, t1.quantity, t2.volume, t2.weight, t2.volume * t1.quantity as total_volume, t1.quantity * t2.weight as total_weight').joins('inner join item_selections t1 on orders.id = t1.order_id, inner join inventory_items t2 on t1.inventory_item_id = t2.id').where(:id => id_val)

使用像这样的自定义选择将其他选择的东西添加为返回对象的属性，因此您可以像引用它们一样是订单对象的字段来引用它们：

Using a custom select like this adds the other things selected as attributes of the returned objects, so you can reference them as though they were fields of the order object:

@total_volume_sum = orders.sum(:total_volume)
@total_weight_sum = orders.sum(:total_weight)

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