application_controller.rb

    if cookies['_old_cookie_name']
      redis = Redis.new
      val = redis.get cookies['_old_cookie_name']
      if val
        old_session = Marshal.load val
        old_session.each{|k,v| session[k] = v}
        redis.incr 'migrated_sessions'
      end
      cookies.delete '_old_cookie_name'
    end