http://wiki.rubyonrails.com/rails/pages/UnderstandingWhatMethodsGoWhere
扩展ActiveRecord示例：
http://wiki.rubyonrails.com/rails/pages/ExtendingActiveRecordExample
增加created_by和updated_by魔术字段：
http://wiki.rubyonrails.com/rails/pages/Howto+Add+created_by+and+updated_by
嵌套集合（树）：
http://threebit.net/tutorials/nestedset/tutorial1.html
http://api.rubyonrails.com/classes/ActiveRecord/Acts/NestedSet/ClassMethods.html
acts_as_threaded插件演示（可制作论坛、邮件线索）：
http://www.railtie.net/plugins/acts_as_threaded/threaded.swf