Question

I’m building an ecommerce application with inventory control and a shopping cart. When the payment has been approved, I want to decrement the quantity of each of the products in the cart. This means, I need to decrement the product quantity column which belongs to line_items which belongs to carts.

I can decrement one through the active record interface like this (in my cart model):

self.line_items[0].product.decrement(:qty)

And also by executing a query directly:

connection.execute("UPDATE products SET qty = qty - 1 WHERE id IN (SELECT product_id FROM line_items WHERE cart_id = #{self.cart_id})")

But both of these methods don’t seem right. The latter queries the db schema directly which is obviously not the best practice. The first method is better but I don’t know how to implement it on all the records.

My models are like so:

class Product < ActiveRecord::Base
end

class Cart < ActiveRecord::Base  
  has_many :line_items
end

class LineItem < ActiveRecord::Base
  belongs_to :cart
  belongs_to :product
end

The schema is:

create_table "carts", :force => true do |t|
  ..
end

create_table "line_items", :force => true do |t|
  ..
  t.integer  "product_id"
  t.integer  "cart_id"
end

create_table "products", :force => true do |t|
  ..
  t.integer  "qty"
end

I am sure there is an elegant way to do this. Can anyone help me please?

Many thanks,
Rim

Answer 1

How about update_counters? http://apidock.com/rails/ActiveRecord/CounterCache/update_counters

class Cart < ActiveRecord::Base  
  has_many :line_items
  has_many :products, :through => :line_items
end

Product.update_counters(cart.products, :qty => -1)
# => UPDATE "products" SET "qty" = COALESCE("qty", 0) - 1 WHERE "products"."id" IN (1, 2)