Uses of Dirty Objects
Developer have some confusion that how they can get the previous value of the field after update.
So here the rails has a way called Dirty objects.
You can found Module Dirty in rails/activerecord/lib/active_record/dirty.rb
This is just for track unsaved attribute changes.
There are few methods in dirty module as below
And it has some constants
DIRTY_SUFFIXES = ['_changed?', '_change', '_will_change!', '_was']
book = Book.find(1)
book.name = 'agiles'
book.changed? # => true
book.name_changed? # => true
book.name_was # => 'rails startup'
book.name_change # => ['rails startup', 'agiles']
book.name = 'agiles with rails'
book.name_change # => ['rails startup', 'agiles with rails']
You can found the details in http://api.rubyonrails.org/classes/ActiveRecord/Dirty.html