Concrete example of WHERE...IN subquery in doctrine 2

EntityRepository.php

<?php
class MyRepository extends EntityRepository
{
    public function whereInSubQuery(User $user)
    {
        $queryBuilder = $this->createQueryBuilder('my_repository');

        $queryBuilder
            ->where(
                $queryBuilder->expr()->in(
                    'my_repository.skill',
                    $this
                        ->_em->createQueryBuilder('subquery_repository')
                        ->select('skill.id')
                        ->from('EntityBundle:Skill', 'skill')
                        ->where('skill.user = :user')
                        ->getDQL()
                )
            )
            ->setParameter(':user', $user);

        return $queryBuilder->getQuery()->getResult();
    }
}