Parent child association mapping and querying

public function &getPageTreePortion($dirId = null) {

        $qb = $this->em->createQueryBuilder();
        if (is_null($dirId)) {
            $qb->select('DP')
                    ->from('Pleomorph\Entities\Core\Domain\PageDirectory', 'DP')
                    ->where('DP.parentDirectory.id IS NULL');
        } else {
            $qb->select('DP')
                    ->from('Pleomorph\Entities\Core\Domain\PageDirectory', 'DP')
                    ->where('DP.parentDirectory.id = ?1');
            $qb->setParameter(1, $dirId);
        }
        $dirs = $qb->getQuery()->getResult();
        return $dirs;
        
    }

SELECT p0_.id AS id0, p0_.title AS title1, p0_.position AS position2, p0_.id_language AS id_language3, p0_.default_page_id AS default_page_id4, p0_.parent_id AS parent_id5 FROM page_directory p0_ WHERE p0_.id IS NULL // or some value

SELECT p0_.id AS id0, p0_.title AS title1, p0_.position AS position2, p0_.id_language AS id_language3, p0_.default_page_id AS default_page_id4, p0_.parent_id AS parent_id5 FROM page_directory p0_ WHERE p0_.parent_id IS NULL  // or some value