gistfile1.rb

# price
# quantity
class LineItem < ActiveRecord::Base
  belongs_to :product
  belongs_to :order

  def calculate_cost
    price * quantity * discount_multiplier
  end

  def discount_multiplier
    # note: I have no idea how the region is set in the app
    discount = Discount.find_by_product_id_and_quantity_and_region(product.id, quantity, order.user.region) 
    discount ? discount.multiplier : 1 # hack, but you get the idea.
  end
end