gistfile1.php

<?php
namespace Moc\GscoreBundle\Document;

use Doctrine\ODM\MongoDB\Mapping\Annotations as MongoDB;

/**
 * @MongoDB\Document
 */
class Athlete
{
    /**
     * @MongoDB\Id
     */
    protected $id;
}

<?php
namespace Moc\GscoreBundle\Document;

use Doctrine\ODM\MongoDB\Mapping\Annotations as MongoDB;

/**
 * @MongoDB\Document
 */
class AthleteScore
{
    /**
     * @MongoDB\Id
     */
    protected $id;

    /**
     * @MongoDB\ReferenceOne(targetDocument="Athlete")
     */
    protected $athlete;
}


$athleteScore = $this->dm->getRepository('GscoreBundle:AthleteScore')->findOneBy(array('athlete.$id' => new \MongoId($athlete)));

$athleteScore = $this->dm->createQueryBuilder('Moc\GscoreBundle\Document\AthleteScore')
            ->field('athlete')->references($athlete)
            ->getQuery()->execute();


        $athleteScore = $this->dm->createQueryBuilder('Moc\GscoreBundle\Document\AthleteScore')
            ->field('athlete.$id')->equals(new \MongoId($athlete->getId()))
            ->getQuery()->execute();