gistfile1.rb

class ActiveRecord::Base
  def self.inherited(klass)
    callbacks = [:before_validation, :before_save, :before_create, :before_update]

    callbacks.each do |callback|
      klass.send(callback) do
        return unless respond_to?(:updated_by) || updated_by.nil?
	self.updated_by = Thread.current[:current_user]
      end
    end
  end
end

class ApplicationController < ActionController::Base
  protect_from_forgery
  before_filter :set_current_user

  private

  def current_user
    # ...
  end

  def set_current_user
    Thread.current[:current_user] = current_user
  end
end