foo.php

<?php

// In the PostRepository

public function getBySlugCategory($slugCategory)
{
  $dql = 'SELECT p FROM Application\Entity\Post p ' .
         'JOIN Application\Entity\PostCategory pc WITH pc.category MEMBER OF p.categories ' .
         'JOIN pc.category c '
         'WHERE c.slug = :slug';
         
         return $entityManager->createQuery($dql)
                              ->setParameter('slug', $slugCategory)
                              ->getResult();
}