app/models/cart.rb

   1 class Cart
   2   attr_reader :items
   3
   4   def initialize
   5     @items = []
   6   end
   7
   8   def add_product(product)
   9     current_item = @items.find {|item| item.product == product}
  10     if current_item
  11       current_item.increment_quantity
  12     else
  13       @items << CartItem.new(product)
  14     end
  15   end
  16
  17   def total_price
  18     @items.sum {|item| item.price}
  19   end
  20
  21 end