gistfile1.php

<?php

// namespace stuff...

class FriendSelectorType extends AbstractType
{

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setRequired(array('user'));
        $resolver->setDefaults(array(
            'class' => 'Acme\WhateverBundle\Document\User',
            'multiple' => false,
            'expanded' => false,
            'property' => 'title',
            'query_builder' => function(Options $options) {
                $user = $options['user'];
                return function(DocumentRepository $dr) use ($user) {
                    return $dr->createQueryBuilder()->field('friends.$id')->equals(new \MongoId($user->getId()));
                };
            }
        ));
    }

    public function getParent()
    {
        return 'document';
    }

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

}