<?php

$qb = $em->createQueryBuilder();

$qb
    ->select('Article', 'Comment')
    ->from('Entity\Article', 'Article')
    ->leftJoin('Article.comment', 'Comment')
    ->where($qb->expr()->eq('Article.title', ':filter_title')
    ->setParameter('filter_title', $some_user_input)
;

$query = $qb->getQuery();

return $query->getResult();