Solve the "uninitialized constant ApplicationController" Problem

Have you just updated your rails version from 2.2.2 to 2.3.5 (or later) and you are getting the strange error message below?

uninitialized constant ApplicationController


This happens because the application controller is no longer called application.rb.
It is now know as application_controller.rb.

There is a rake task to do the renaming automatically for you. Simply run:

rake rails:update