error_messages_on

BIG GOTCHA!

1. don't use redirect_to in the controller otherwise the view wont get, i.e. display the error message. user ender instead.

2. error_messages_on wants the name of the instance variable used in the controller, ie if you use,

error_messages_on 'instance_variable'

then make sure you use
@instance_variable in your controller code. Its this vairable that has the method @instance_vaiable.errors which error_on_messages uses.