## line_item.rb

class LineItem < ActiveRecord::Base
  belongs_to :product
  def self.for_product(product)
    item = self.new
    item.quantity = 1
    item.product = product
    item.unit_price = product.price
  end
end



## product.rb
class Product < ActiveRecord::Base
  validates_presence_of :title, :description, :image_url
  validates_numericality_of :price
  validates_uniqueness_of :title
  validates_format_of :image_url,
    :with     => %r{\.(gif|jpg|png)$}i,
    :message  => "must be a URL for a GIF, JPG or PNG image"

  protected
  def validate
    errors.add(:price, "should be positive") unless price.nil? || price>=0.01
  end

  # Return a list of products we can sell (which means they have to be
  # available). Show the most recent available first.
  def self.salable_items
    find( :all,
          :conditions =>  "date_available <= now()",
          :order      =>  "date_available desc")
  end
end

## cart.rb
class Cart
  attr_reader :items
  attr_reader :total_price

  def initialize
    @items = []
    @total_price = 0.0
  end
  def add_product(product)
    @items << LineItem.for_product(product)
    @total_price += product.price
  end
end