I've read the following post Calling AppModel function in AppController for cakephp where the OP has asked if they can use a function from their AppModel inside AppController.
The answer given doesn't really address the question (since it talks about using the AppModel in another model and not a controller).
So, is it acceptable to do something like this inside the AppController?
$this->loadModel('AppModel');
$this->AppModel->my_function();
The reasoning why someone may want to do this is because the AppController is run on every request. If you need to do something that involves your DB on every request, the logic for that may go in AppModel.php but be executed via AppController.php
Please note this applies to Cake 2.0 (not 3) as it is a legacy application we are dealing with.