Controller.php

config.yml

doctrine:
    dbal:
        driver:   %database_driver%
        host:     %database_host%
        dbname:   %database_name%
        user:     %database_user%
        password: %database_password%
        charset:  UTF8

    orm:
        auto_generate_proxy_classes: %kernel.debug%
        entity_managers:
            default:
                auto_mapping: true
                dql:
                    string_functions:
                        rand_string: JStout\MainBundle\Component\Doctrine\DQL\Rand

gistfile1.php

<?php

[...] 

        $qb = $this->entityManager->createQueryBuilder();

        $qb->select('o, oz')
           ->from('JStoutMainBundle:Offer', 'o')
           ->leftJoin('o.zones', 'oz')
           ->orderBy('RAND()');

        $query = $qb->getQuery();

        $query->setMaxResults(1);

        $offer = $query->getSingleResult();

Rand.php

<?php
namespace JStout\MainBundle\Component\Doctrine\DQL;

use Doctrine\Common\Lexer;
use Doctrine\ORM\Query\AST\Functions\FunctionNode;
use Doctrine\ORM\Query\Parser;
use Doctrine\ORM\Query\SqlWalker;

/**
* RandFunction ::= "RAND" "(" ")"
*/
class Rand extends FunctionNode
{
    public function parse(Parser $parser)
    {
        $parser->match(Lexer::T_IDENTIFIER);
        $parser->match(Lexer::T_OPEN_PARENTHESIS);
        $parser->match(Lexer::T_CLOSE_PARENTHESIS);
    }

    public function getSql(SqlWalker $sqlWalker)
    {
        return 'RAND()';
    }
}