doctrine_mongodb:
    connections:
        default:
            server: mongodb://localhost:27017
            options:
                connect: true
    default_database: symfony2
    document_managers:
        default:
            auto_mapping: true

<?php
namespace HALLHelloWorldBundleDocument;
use FOSUserBundleDocumentUser as BaseUser;
use DoctrineODMMongoDBMappingAnnotations as MongoDB;

/**
 * @MongoDBDocument
 */
class User extends BaseUser
{
    /** @MongoDBId(strategy="auto") */
    protected $id;

    public function __construct()
    {
        parent::__construct();
        // your own logic
    }
}

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

AnnotationRegistry::registerFile(__DIR__.'/../vendor/doctrine-mongodb-odm/lib/Doctrine/ODM/MongoDB/Mapping/Annotations/DoctrineAnnotations.php');

DoctrineODMMongoDBMappingDriverAnnotationDriver::registerAnnotationClasses();

use DoctrineODMMongoDBConfiguration;

.........

$configuration = new Configuration();
$driver = $configuration->newDefaultAnnotationDriver($path_to_docs);
$driver->registerAnnotationClasses();

<?php

use DoctrineCommonAnnotationsAnnotationRegistry;
use DoctrineODMMongoDBMappingDriverAnnotationDriver; <-- add this line

$loader = require __DIR__.'/../vendor/autoload.php';

if (!function_exists('intl_get_error_code')) {
    require_once __DIR__.'/../vendor/symfony/symfony/src/Symfony/Component/Locale/Resources/stubs/functions.php';

    $loader->add('', __DIR__.'/../vendor/symfony/symfony/src/Symfony/Component/Locale/Resources/stubs');
}

AnnotationRegistry::registerLoader(array($loader, 'loadClass'));

AnnotationDriver::registerAnnotationClasses();  <-- add this line

return $loader;