app/models/cart_item.rb

   1 class CartItem
   2   attr_reader :product, :quantity
   3
   4   def initialize(product)
   5     @product = product
   6     @quantity = 1
   7   end
   8
   9   def increment_quantity
  10     @quantity += 1
  11   end
  12
  13   def title
  14     @product.title
  15   end
  16
  17   def price
  18     @product.price * @quantity
  19   end
  20
  21 end