Symfony2 / Doctrine How to store the number of related objects inside an object

I installed a package that has a test object that contains several testQuestion objects, each of which is a question and this answer (or 0 if there is no answer). From the branch I want to get information from the test object to say how many questions there are and how many have been answered.

I created a request to pull this from db, and in Test Entity I created 2 new properties to store the number of questions and answer to the number. I created a TestRepository inside which the request is located. The Test object checks to see if the object has a given value and if it does not load it when necessary, since I will not always need this information.

However, I was fixated on how to associate the repository code with the test object, both for calling the repo function and for the repo function, in order to store the values ​​in the corresponding test object.

Acme / Quizbundle / Test / test.php

namespace Acme\QuizBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Acme\QuizBundle\Entity\TestRepository;

/**
 * @ORM\Entity(repositoryClass="Acme\QuizBundle\Entity\TestRepository")
 * @ORM\Table(name="test")
 */
class Test    {
protected $numQuestions = null;
protected $numQuestionsAnswered = null;

public function getNumQuestionsAnswered () {
    if (is_null($this->numQuestionsAnswered)) {
        $repository = $this->getEntityManager()->getRepository('\AcmeQuizBundle\Test');
        $values = $repository->calculateNumQuestions();
    }
    return $this->numQuestionsAnswered;
}

Acme / Quizbundle / Test / TestRepository.php (there is a corresponding method for getNumQuestions ())

namespace Acme\QuizBundle\Entity;

use Doctrine\ORM\EntityRepository;

class TestRepository extends EntityRepository {

private function calculateNumQuestions() {

    $qb = $this->getEntityManager()
                ->createQueryBuilder();

    $query = $this->getEntityManager()->createQueryBuilder()
                        ->select('COUNT(id)')
          ->from('testquestion', 'tq')
          ->where('tq.test_id = :id')
          ->setParameter('id', $this->getId())
          ->getQuery();

    $result = $query->getSingleScalarResult();
    var_dump($result);
    }