My question heavily depends on this: Doctrine: Query only where relationship doesn't exist?
I wonder how the very same thing can be accomplished using Doctrine2. I am especially interested in good practices. Do I have to use the QueryBuilder or is it possible to use a findBy on a repository?
Eventually I used QueryBuilder like this (example from the question linked above):
// $entitiyManager: Doctrine EntityManager instance
$qb = $entityManager->createQueryBuilder();
$x = $qb->expr();
$qb->select('article')
->from('Article', 'article')
->leftJoin('article.category', 'category')
->where($x->isNotNull('category.id');