Some Doctrine 2 Best Practices got here - http://www.uvd.co.uk/blog/some-doctrine-2-best-practices/

dont-do-this.php

<?php

$articles = $article_repository->findAll();

foreach($articles as $article)
{
    echo $article->getTitle();
    echo count($article->getComments());
}

fetch-join-instead.php

<?php

//$em is the entity manager
$qb = $em->createQueryBuilder();

$qb
    ->select('Article', 'Comment')
    ->from('Entity\Article', 'Article')
    ->leftJoin('Article.comment', 'Comment')
;

$query = $qb->getQuery();

return $query->getResult();

partial-select.php

<?php

//$em is the entity manager
$qb = $em->createQueryBuilder();

// Wasteful, will select all the properties (columns) for the entity
$qb
    ->select('Article')
    ->from('Entity\Article')
;

// Better, but won't provide a structure representing our object graph (returns a flat array)
$qb
    ->select('Article.title')
    ->from('Entity\Article')
;

// Best (although fragile if trying to manipulate/persist objects later)
$qb
    ->select('partial Article.{title}')
    ->from('Entity\Article')
;

// You can select multiple properties in the partial statement
$qb
    ->select('partial Article.{id,title,summary}')
    ->from('Entity\Article')
;

prepared-statements.php

<?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();