我修改了某個attribute,但還沒儲存,要如何取得修改前的值?

硬幹的作法就是在改值之前先用個local variable存起來,但是否有更優美的解法呢?

有!就用 attribute_name_was

請看以下範例:

	ruby-1.9.2-p290 :003 > u.email  
	 => "basdfasdf@asdawdfas.com"   
	ruby-1.9.2-p290 :004 > u.email="xxxxx@xxxxxx.xx"  
	 => "xxxxx@xxxxxx.xx"   
	ruby-1.9.2-p290 :005 > u.email  
	 => "xxxxx@xxxxxx.xx"   
	ruby-1.9.2-p290 :006 > u.email_was  
	 => "basdfasdf@asdawdfas.com"   

但是save以後就會被洗掉:
ruby-1.9.2-p290 :007 > u.save
=> true
ruby-1.9.2-p290 :008 > u.email_was
=> “xxxxx@xxxxxx.xx”

以下是追蹤attributes變化的methods(來自RailsCasts
p.changed?
p.changed
p.changes
p.name_changed?
p.name_was
p.name_change
p.name_will_change!