cakephp delete casade with sub-categories

cakephp delete casade with sub-categories

I have 3 tables (I've shortened the names for this example removing the
plural/singular cakephp conventions):
Main (has many Sub1)
Sub1 (belongs to main, hasmany Sub2)
Sub2 (belongs to sub1)
When I delete a category from Main, all of its dependent sub1/sub2 items
are properly deleted using $this->Main->delete($id, true) call.
However I'm not sure how to delete an item from Sub1 and remove all it's
sub2 properties. I'm new with cakephp so bare with me. The controller
needs to manipulate all three levels of the tables. I tried:
$this->loadModel('Sub1');
$res = $this->Sub1->delete($id, true);
But it is not picking up the model bindings to Sub2 and deleting them.
What is the proper convention for doing this kind of thing in cake and/or
what am I doing wrong?