http://wiki.rubyonrails.com/rails/pages/UnderstandingWhatMethodsGoWhere
擴(kuò)展ActiveRecord示例：
http://wiki.rubyonrails.com/rails/pages/ExtendingActiveRecordExample
增加created_by和updated_by魔術(shù)字段：
http://wiki.rubyonrails.com/rails/pages/Howto+Add+created_by+and+updated_by
嵌套集合（樹）：
http://threebit.net/tutorials/nestedset/tutorial1.html
http://api.rubyonrails.com/classes/ActiveRecord/Acts/NestedSet/ClassMethods.html
acts_as_threaded插件演示（可制作論壇、郵件線索）：
http://www.railtie.net/plugins/acts_as_threaded/threaded.swf