http://chriskief.com/2012/12/17/symfony2-vs-django-models/

BlogRepository.php

<?php

namespace NewCoInc\AppBundle\Entity;

use Doctrine\ORM\EntityRepository;

class BlogRepository extends EntityRepository
{
	public function getTags()
	{
		$blogTags = $this->createQueryBuilder('b')
			->select('b.tags')
			->getQuery()
			->getResult();

		$tags = array();
		foreach ($blogTags as $blogTag)
		{
			$tags = array_merge(explode(",", $blogTag['tags']), $tags);
		}

		foreach ($tags as &$tag)
		{
			$tag = trim($tag);
		}

		return $tags;
	}
}