gistfile1.rb

  def add_discount(promo_code)
    discount = Discount.find_by_code(promo_code)
    if discount && discount.applicable?(self)
      if discount.customer_exclusive?
        discount_assignment = discount.assignments.find_by_user_id(user.id)
        discount_assignment.update_attributes(:sales_order_id => self.id)
      else
        discount_assignments.create(:discount => discount)
      end
      
      save
      reload
      discount
    else
      errors.add_to_base "The code you entered is invalid."
      nil
    end
  end

  def validate_applied_discounts
    discount_assignments.select { |a|
      ! a.discount.applicable?(self)
    }.each { |a|
      a.destroy
    }
  end
  
  def calculate_discount_total
    self.discount_total = discounts(true).inject(0) do |sum,discount|
      sum += discount.calculate(self)
    end
  end

  ###
  #  in discount:

  def compatible?(*discounts)
    discounts.length == 0 || (can_use_with_other_discounts? && discounts.all?{|d| d.can_use_with_other_discounts? })
  end