Can't retrieve value of a field of type @Collection using DoctrineMongoDBBundle

I have the following definition of a MongoDB Document (I stripped down parts of the class for brevity):

// src/Acme/UserBundle/Document/User.php
namespace Acme\UserBundle\Document

use Doctrine\ODM\MongoDB\Mapping\Annotations as MongoDB;
use Symfony\Component\Security\Core\User\UserInterface;
use Symfony\Component\Security\Core\User\EquatableInterface;

use Symfony\Component\Validator\Constraints as Assert;
use Doctrine\Bundle\MongoDBBundle\Validator\Constraints\Unique as MongoDBUnique;

/**
 * @MongoDB\Document()
 * @MongoDBUnique(fields="email", message="This email is already registered")
 */
class User implements UserInterface, EquatableInterface
{
    /**
     * @MongoDB\Id
     */
    protected $id;

    /**
     * @MongoDB\String
     * @Assert\Email(checkMX=true)
     */
    protected $email;

    /**
     * @MongoDB\Collection
     */
    protected $languages = array();
}

Next I generate the setters and getters with:

php app/console doctrine:mongodb:generate:documents UserBundle 

I use the User class as usual, working everything fine, I can store instances of user that get correctly stored in mongodb.

Querying with the mongodb's console, I can see how the object is stored and that languages stores an array:

> db.User.find()
{ "_id" : ObjectId("51c2da8d8b4d24315e9ec3d2"), "email" : "test@test.com", "languages" : [  "en",  "fr" ] }

But when I try to retrieve the data:

$dm = $this->get('doctrine_mongodb')->getManager();
$user = $dm->getRepository('UserBundle:User')->findOneByEmail('test@test.com');

$user->getEmail() returns the E-mail correctly but $user->getLanguages() always return an empty array.

EDIT:

I managed to run a query using a CLI Command for app/console:

$dm = $this->getContainer()->get('doctrine_mongodb')->getManager();
$user = $dm
    ->getRepository('UserBundle:User')
    ->findOneByEmail('test@test.com');

    print_r($user->getLanguages());

And the output of the print_r command was correct:

Array
(
    [0] => en
    [1] => fr
    [2] => pt
    [3] => es
)

EDIT2:

And after that I tried the application again and it worked normally. I don't know why.

I just had this problem and take a quick look at doctrine mongodb implementation.

Doctrine creates some hydrators classes at runtime (thanks to the class Doctrine\ODM\MongoDB\Hydrator\HydratorFactory) and store the result in the cache. The cache is built on the first request then the cache is used. This is why you had this strange behaviour and resolution.

A simple reset of the cache solves this problem:

php app/console cache:clear