Transactions with Model

Hi, everyone knows how to use transaction, right?

Code:

<?php

$this->db->transStart();
$this->db->query('AN SQL QUERY...');
$this->db->query('ANOTHER QUERY...');
$this->db->query('AND YET ANOTHER QUERY...');
$this->db->transComplete();

But the truth is will be more useful to use transaction with the model. Something Like This:

Code:

$UserModel->db->transStart();

$UserModel->save($user);
$ProfileModel->save($profile);

$UserModel->db->transComplete();

But this approach seems not work. Anyone advice?