Doctrine retrieve entity data with one to many association - doctrine-orm

Question

i want to fetch the data of a menu including its categories in a custom repository by a dql-statement, but it doesn't return the associated entities. I need it as json data, so i added the hydration mode Query::HYDRATE_ARRAY to the function call.
/**
* #ORM\Entity(repositoryClass="Company\Repository\Doctrine\MenuRepository")
* #ORM\Table(name="menu")
*/
class Menu {
/**
* #var \Company\Entity\MenuCategory[]
*
* #ORM\OneToMany(targetEntity="Company\Entity\MenuCategory", mappedBy="menu")
*/
protected $categories;
}
/**
* #ORM\Entity(repositoryClass="Company\Repository\Doctrine\MenuCategoryRepository")
* #ORM\Table(name="menu_category")
*/
class MenuCategory {
/**
* #var \Company\Entity\Menu
*
* #ORM\ManyToOne(targetEntity="Company\Entity\Menu", inversedBy="categories")
* #ORM\JoinColumn(nullable=false)
*/
protected $menu;
}
class MenuRepository extends EntityRepository implements MenuRepositoryInterface {
public function findById($id, $hydration = Query::HYDRATE_OBJECT) {
$queryBuilder = $this->createQueryBuilder('menu')
->leftJoin('menu.categories', 'categories')
->where('menu.id = :menuId')
->setParameter('menuId', $id);
return $queryBuilder->getQuery()->getSingleResult($hydration);
}
}
The result looks like that:
array(4) {
["id"]=>int(1)
["name"]=> string(6) "Test"
}