Duplicate queries while eager loading multiple associations

aris · September 17, 2012, 9:08am

Hello all,

given the following models and console log:

gist.github.com

https://gist.github.com/krupenik/3735116

gistfile1.txt

irb(main):055:0> o = Order.last
  Order Load (0.4ms)  SELECT "orders".* FROM "orders" ORDER BY "orders"."id" DESC LIMIT 1
=> #<Order id: 143, customer_id: 13, session_id: "5c7f3b0268022483ed1c87ff9fb5304a", state: nil, discount: #<BigDecimal:7fe419d863f0,'0.0',9(18)>, balance: #<BigDecimal:7fe419d86328,'0.0',9(18)>, notes: nil, shipping_method: nil, shipping_details: nil, created_at: "2012-09-16 17:27:39", updated_at: "2012-09-16 21:09:56">
irb(main):056:0> oi = o.order_items.includes(item: :photos)
  OrderItem Load (0.4ms)  SELECT "order_items".* FROM "order_items" WHERE "order_items"."order_id" = 143
  Item Load (0.2ms)  SELECT "items".* FROM "items" WHERE "items"."id" IN (227)
  Photo Load (0.1ms)  SELECT "photos".* FROM "photos" WHERE "photos"."item_id" IN (227)
=> [#<OrderItem id: 302, order_id: 143, item_id: 227, item_price: #<BigDecimal:7fe4198f21b8,'0.5E2',9(18)>, quantity: 1, size: 95, type: "rent", starts_on: "2012-09-17", ends_on: "2012-09-18", created_at: "2012-09-16 20:03:33", updated_at: "2012-09-16 21:09:56">]
irb(main):057:0> o.order_items
  OrderItem Load (0.4ms)  SELECT "order_items".* FROM "order_items" WHERE "order_items"."order_id" = 143

This file has been truncated. show original

item.rb

class Item < ActiveRecord::Base
  has_many :order_items
  has_many :orders, through: :order_items

  attr_accessible :rent_price, :sale_price
end

order.rb

class Order < ActiveRecord::Base
  has_many :order_items, dependent: :destroy

  def total
    order_items.map(&:price).reduce(:+)
  end
end

There are more than three files. show original

I do not understand why the latter statements (o.order_items and
o.total)
yield sql queries.
Have I missed something in AR documentation? What should be done to get
rid
of redundant sql queries?

dkathrens77 · September 17, 2012, 9:14am

On Monday, September 17, 2012 2:45:05 AM UTC+1, Dennis K. wrote:

oi = o.order_items.includes(item: :photos)

does not mutate o.order_items, adding the includes to it: it returns a
new
array of objects that has the extra options applied.

You do o = Order.includes(order_items: {item: photos}).last in which
case

o.order_items should not execute any queries

Fred

dkathrens77 · September 17, 2012, 4:19pm

thanks a lot, that did it