Sometimes we face a situation where after changing the value of an attribute of an object we need the previous or the unchanged value for some purpose. We generally store the previous value in a variable to be used later. Instead of this approach we can use these methods that are provided by ActiveModel::Dirty class.

Example:

user = User.first
user.name # => John
user.changed?       # => false



user.name = 'John Snow'
user.changed?       # => true
user.name_changed?  # => true
user.name_was       # => John
user.name_change    # => ["John", "John Snow"]
user.changes # => {"name"=>["John", "John Snow"]}

We can see the changes even after saving the object.

user.save
user.changes       # => {}
user.previous_change # => {"name"=>["John", "John Snow"]}

We can reset the changes by reloading the model.

user.reload
user.previous_changes # => {}

Based on the above methods we can write callbacks in the model.

class User < ActiveRecord::Base
  before_update :set_password_edited,  if: encrypted_password_changed?

    def set_password_edited
    write_attribute(:password_edited_on, Time.now)
  end

end