app/models/line_item.rb

   1 class LineItem < ActiveRecord::Base
   2   belongs_to :order
   3   belongs_to :product
   4
   5   def self.from_cart_item(cart_item)
   6     li = self.new
   7     li.product      = cart_item.product
   8     li.quantity     = cart_item.quantity
   9     li.total_price  = cart_item.price
  10     li
  11   end
  12 end