Symfony2/Doctrine How to store count of related objects within an Entity
===

I have set up a bundle that has a test object which holds a number of testQuestion objects each of which is a question and the given answer (or 0 if no answer). From twig I want to be able to get the information from the test object to say how many questions there are and how many have been answered. 

I have created a query to pull this out of the db, and in the Test Entity I have created 2 new properties to store the number of questions and the number answered. I have created a TestRepository inside which the query resides. The Test object checks to see if the object has the value set and if not loads it when needed as I won't always need this information.

However I'm stuck on how to link the repository code to the test object, both to call the repo function and for the repo function to save the values to the relevant 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's a matching 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);
        }