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_accessor :bar
 end

I would like to be able to track changes to 'bar', with methods like 'bar_changed?', as provided by ActiveModel Dirty. The problem is that when I try to implement Dirty on this object, as described in the docs, I'm getting an error as both ActiveRecord and ActiveModel have defined define_attribute_methods, but with different number of parameters, so I'm getting an error when trying to invoke define_attribute_methods [:bar].

I have tried aliasing define_attribute_methods before including ActiveModel::Dirty, but with no luck: I get a not defined method error.

Any ideas on how to deal with this? Of course I could write the required methods manually, but I was wondering if it was possible to do using Rails modules, by extending ActiveModel functionality to attributes not handled by ActiveRecord.

Answer 1

I'm using the attribute_will_change! method and things seem to be working fine.

It's a private method defined in active_model/dirty.rb, but ActiveRecord mixes it in all models.

This is what I ended up implementing in my model class:

def bar
  @bar ||= init_bar
end
def bar=(value)
  attribute_will_change!('bar') if bar != value
  @bar = value
end
def bar_changed?
  changed.include?('bar')
end

The init_bar method is just used to initialise the attribute. You may or may not need it.

I didn't need to specify any other method (such as define_attribute_methods) or include any modules. You do have to reimplement some of the methods yourself, but at least the behaviour will be mostly consistent with ActiveModel.

I admit I haven't tested it thoroughly yet, but so far I've encountered no issues.

Answer 2

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

Answer 3

I figured out a solution that worked for me...

Save this file as lib/active_record/nonpersisted_attribute_methods.rb: https://gist.github.com/4600209

Then you can do something like this:

require 'active_record/nonpersisted_attribute_methods'
class Foo < ActiveRecord::Base
  include ActiveRecord::NonPersistedAttributeMethods
  define_nonpersisted_attribute_methods [:bar]
end

foo = Foo.new
foo.bar = 3
foo.bar_changed? # => true
foo.bar_was # => nil
foo.bar_change # => [nil, 3]
foo.changes[:bar] # => [nil, 3]

However, it looks like we get a warning when we do it this way:

DEPRECATION WARNING: You're trying to create an attribute `bar'. Writing arbitrary attributes on a model is deprecated. Please just use `attr_writer` etc.

So I don't know if this approach will break or be harder in Rails 4...