FormType.php

<?php
namespace Acme\MessageBundle\FormType;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;
use Symfony\Component\Security\Core\SecurityContext;
use Doctrine\ODM\MongoDB\DocumentRepository;

class FriendMessageFormType extends AbstractType
{
    private $user;

    public function __construct(SecurityContext $security_context)
    {
        $this->user = $security_context->getToken()->getUser();
    }

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $user_id = $this->user->getId();

        $form_options = [
            'class' => 'Acme\MessageBundle\Document\User',
            'multiple' => false,
            'expanded' => false,
            'property' => 'title',
            // 'attr' => ['class' => 'select2'],
            'query_builder' => function(DocumentRepository $dr) use ($user_id) {
                return $dr->createQueryBuilder()->field('friends')->equals(new \MongoId($user_id));
            }
        ];

        $builder->add('friend', 'document', $form_options);

        $builder
            ->add('subject', 'text')
            ->add('body', 'textarea')
        ;
    }

    public function getName()
    {
        return 'acme_friend_message';
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
    }
}