after_commit_callbacks.rb

module AfterCommitCallbacks
  def self.included(base)
    base.send(:extend, ObserverCallbacks) unless base.respond_to?(:define_model_callbacks_for_observers)

    [:create, :update, :destroy].each do |action|
      base.send(:define_model_callbacks_for_observers, :"commit_on_#{action}", :only => :after)
    end

    base.send(:attr_accessor, :newly_created)
    base.send(:before_validation, AfterCommitCallbacks::Handlers)
    base.send(:after_commit, AfterCommitCallbacks::Handlers)
  end

  module Handlers
    def self.before_validation(record)
      record.newly_created = record.new_record?

      true
    end

    def self.after_commit(record)
      action = record.destroyed? ? "destroy" : (record.newly_created ? "create" : "update")

      record.run_callbacks :"commit_on_#{action}"

      true
    rescue Exception => ex
      p ex
      raise ex
    end
  end
end

observer_callbacks.rb

module ObserverCallbacks
  # Calls define_model_callbacks and notify observers when called
  def define_model_callbacks_for_observers(*args)
    types = Array.wrap(args.extract_options![:only] || [:before, :around, :after])

    callbacks = define_model_callbacks(*args)

    callbacks.each do |callback|
      types.each do |filter|
        set_callback(callback, filter) do
          notify_observers :"#{filter}_#{callback}"
          true
        end
      end
    end
  end
end

after_update_on_update is triggered for new records.

to reproduce the issue

  class User < ActiveRecord::Base
  end
  class UserObserver < ActiveRecord::Observer
    def after_commit_on_create 
      puts 'on create'
    end
    def after_commit_on_update
      puts 'on update'
    end
  end

  User.new.save(:validate => false)

If I change the code it worked for me. But I am not sure will it going to break in some other place. So, Please review my changes,

https://gist.github.com/PratheepV/228d0ab4abeb490370e3

Here I just changed before_validate to before_save.

Thanks,
Pratheepv