Undefined method stdClass::user() error when using CakePHP Auth

Hi there,

I'm fairly new to CakePHPand am building a site using the Auth component. A couple of times I have tried to do things with this component which have caused the error

Fatal error: Call to undefined method stdClass::user() in /ftphome/site/app/controllers/users_controller.php on line 395

The line it refers to in this case is

$this->User->read(null, $this->Auth->user('id'));

This error does not disappear when I revert the code back to how it was before the error and I only seem to be able to get rid of it by removing some files on the server (I'm not sure which files, when I tried removing all files in the tmp directory the error persisted so I removed the entire site and restored from the latest svn revesion.

In this particular case I think I caused the error by putting the following code in app_controller

class AppController extends Controller {
    function beforeRender() {
        $this->set('test', $this->Auth->user());
    }
}

Which I copied from this thread http://groups.google.com/group/cake-php/browse_thread/thread/ee9456de93d2eece/cff6fe580d13622b?lnk=gst&q=auth

A previous time I caused the issue by attempting to update the Auth user details after updating the user in the database.

I can see I'm somehow removing the user object from the Auth object but I can't understand why I need to delete files in the site to get it back or how the code above removes it - any help would be much appreciated.

Edit

It looks like in the case I mentioned above, the problem was the app_controller.php file which I'd copied into my app/controllers directory. Just having the file with an empty class declaration causes this error - can anyone provide further insight?

Further edit

I've realised I've been a bit silly and the problem was caused by me putting app_controller.php in /app/controllers/app_controller.php when there was already one in /app/app_controller.php - thanks for the input though Andy, it helps me understand a bit more what was happening.