order.rb

class Order < ActiveRecord::Base
  has_many :line_items

  before_validation :initialize_subtotal  # good
  before_save :recalculate_subtotal # bad

private

  def recalculate_subtotal
    self.subtotal = self.line_items.map(&:subtotal).sum
  end

  def initialize_subtotal
    self.subtotal ||= 0
  end
end