Doctrine Query Help

doctrine.php

<?php

$qb2 = $this->createQueryBuilder('a2')
    ->select('a2')
    ->innerJoin('a2.registrations', 'appointment_registrations')
    ->where('a2.id = a1.id') // This does not work :(
;

$qb = $this->createQueryBuilder('a1');
$qb->where($qb->expr()->not($qb->expr()->exists($qb2->getDql())));

query.sql

SELECT a1.* 
FROM appointments a1 WHERE 
NOT 
	( EXISTS
		(
			SELECT a2.* FROM appointments a2 
			INNER JOIN appointment_registrations r on r.appointmentId = a2.id
			WHERE a2.id = a1.id
		)
	)