gistfile1.rb

class Sku
  attr_reader :identifier
  attr_reader :price
end

class Cart
  many :cart_items
  
  def quantity
    cart_items.inject(0) { |quantity, item| quantity += item.quantity }
  end

  def sum
    cart_items.inject(0) { |sum, item| sum += item.sum }
  end
end

class CartItem
  belongs_to :sku
  attr_reader :quantity

  def sum
    quantity * sku.price
  end
end