Subscriber.php

<?php
namespace Acme\WhateverBundle\FormSubscriber;

use Symfony\Component\Form\Event\DataEvent;
use Symfony\Component\Form\FormFactoryInterface;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\Form\FormEvents;
use Doctrine\ODM\MongoDB\DocumentRepository;
use Symfony\Component\Security\Core\SecurityContext;

/**
 * Only add user's friends as possible choices
 */
class OrganizerEventsListener implements EventSubscriberInterface
{
    /**
     * @var FormFactoryInterface
     */
    private $factory;

    private $user;

    /**
     * @param factory FormFactoryInterface
     */
    public function __construct(FormFactoryInterface $factory, SecurityContext $security_context)
    {
        $this->factory = $factory;
        $this->user = $security_context->getToken()->getUser();
    }

    public static function getSubscribedEvents()
    {
        return [FormEvents::PRE_SET_DATA => 'preSetData'];
    }

    /**
     * @param event DataEvent
     */
    public function preSetData(DataEvent $event)
    {
        $data = $event->getData();
        $form = $event->getForm();
        $user_id = $this->user->getId();

        $form_options = [
            'class' => 'Acme\WhateverBundle\Document\User',
            'multiple' => false,
            'expanded' => false,
            'property' => 'title',
            'query_builder' => function(DocumentRepository $dr) use ($user_id) {
                return $dr->createQueryBuilder()->field('friends.$id')->equals(new \MongoId($user_id));
            }
        ];

        $form->add($this->factory->createNamed('friend', 'document', null, $form_options));
    }

}