cart.rb

class Cart < ActiveRecord::Base

  has_many :cart_lines
  accepts_nested_attributes_for :cart_lines, allow_destroy: true
....
end

gistfile1.txt

# the last cart has 2 cart_lines
# let's update these attributes
# as they were copied from our PATCH /carts/1 log

CartLine.pluck :quantity
   (2.1ms)  SELECT "cart_lines"."quantity" FROM "cart_lines"
=> [1, 1]



attrs = {"cart_lines_attributes"=>{"0"=>{"quantity"=>"2", "id"=>"1"}, "1"=>{"quantity"=>"5", "id"=>"2"}}}

a=Cart.last
  Cart Load (1.9ms)  SELECT "carts".* FROM "carts" ORDER BY "carts"."id" DESC LIMIT 1
=> #<Cart id: 1, quantity: 2, old_price: #<BigDecimal:bba0b660,'0.0',9(18)>, main_price: #<BigDecimal:bba0b5fc,'0.11E0',9(18)>, cost_price: #<BigDecimal:bba0b598,'0.0',9(18)>, created_at: "2013-08-27 18:16:23", updated_at: "2013-08-27 18:23:10">


a.update_attributes! attrs
   (0.9ms)  BEGIN
  CartLine Load (1.0ms)  SELECT "cart_lines".* FROM "cart_lines" WHERE "cart_lines"."cart_id" = $1 AND "cart_lines"."id" IN (1, 2)  [["cart_id", 1]]
  CartLine Load (2.3ms)  SELECT "cart_lines".* FROM "cart_lines" WHERE "cart_lines"."cart_id" = $1  [["cart_id", 1]]
   (0.7ms)  COMMIT
=> true


irb(main):044:0> a.cart_lines[0].quantity
=> 1
irb(main):045:0> a.cart_lines[1].quantity
=> 1
irb(main):046:0> a.cart_lines(true)[1].quantity
  CartLine Load (1.8ms)  SELECT "cart_lines".* FROM "cart_lines" WHERE "cart_lines"."cart_id" = $1  [["cart_id", 1]]
=> 1
irb(main):047:0> a.cart_lines(true)[0].quantity
  CartLine Load (1.6ms)  SELECT "cart_lines".* FROM "cart_lines" WHERE "cart_lines"."cart_id" = $1  [["cart_id", 1]]
=> 1