order.rb

  def total
    line_items.map {|i| i.price.to_f * i.quantity.to_i }.sum *
      (1.0 - quantity_discount)
  end

  def remaining_total
    total - payments.map(&:amount).compact.sum
  end
  
  def quantity_discount
    return 0 unless catalog.quantity_discounts.present?
    pieces = line_items.map {|i| i.quantity.to_i}.sum
    catalog.quantity_discounts.select do |qty, discount|
      qty < pieces
    end.max.try(:[], 1) || 0
  end