Track dirty for not-persisted attribute in an ActiveRecord object in rails

Question

I have an object that inherits from ActiveRecord, yet it has an attribute that is not persisted in the DB, like:

 class Foo < ActiveRecord::Base
   attr_acces         


        

Answer 1

ActiveRecord has the #attribute method (source) which once invoked from your class will let ActiveModel::Dirty to create methods such as bar_was, bar_changed?, and many others.

Thus you would have to call attribute :bar within any class that extends from ActiveRecord (or ApplicationRecord for most recent versions of Rails) in order to create those helper methods upon bar.

Edit: Note that this approach should not be mixed with attr_accessor :bar

Edit 2: Another note is that unpersisted attributes defined with attribute (eg attribute :bar, :string) will be blown away on save. If you need attrs to hang around after save (as I did), you actually can (carefully) mix with attr_reader, like so:

attr_reader :bar
attribute :bar, :string

def bar=(val)
  super
  @bar = val
end