How to make transactions in Doctrine 2 using Symfony 2

transaction.php

<?php

// Snippet for Symfony 2 application that uses Doctrine 2 to handle transactions
// It uses the names of the objects/doctrine repositories from the Beta 4 Manual of Symfony 2.

// Get the entity manager
$em = $this->getDoctrine()->getEntityManager();

// suspend auto-commit
$em->getConnection()->beginTransaction();

// Try and make the transaction
try {
    // Get the account
    $account = $this->getDoctrine()->getRepository('AcmeStoreBundle:Account')->find($id);

    // Lower balance
    $account->setBalance($account->getBalance() - 100);

    // Save account
    $em->persist($account);
    $em->flush();

    // Try and commit the transaction
    $em->getConnection()->commit();
} catch (Exception $e) {
    // Rollback the failed transaction attempt
    $em->getConnection()->rollback();
    throw $e;
}