Use the entity form type in Silex

app.php

<?php

$app['form.extensions'] = $app->share($this->extend('form.extensions', function ($extensions) {
    $managerRegistry = new ManagerRegistry(null, array(), array('doctrine_orm.em'), null, null, $app['doctrine_orm.proxies_namespace']);
    $managerRegistry->setContainer($app);
    $extensions[] = new DoctrineOrmExtension($managerRegistry);

    return $extensions;
}));

ManagerRegistry.php

<?php

namespace Umpirsky\Doctrine\Common\Persistance;

use Doctrine\Common\Persistence\AbstractManagerRegistry;
use Silex\Application;

/**
 * References Doctrine connections and entity/document managers.
 *
 * @author Саша Стаменковић <[email protected]>
 */
class ManagerRegistry extends AbstractManagerRegistry
{
    /**
     * @var Application
     */
    protected $container;

    protected function getService($name)
    {
        return $this->container[$name];
    }

    protected function resetService($name)
    {
        unset($this->container[$name]);
    }

    public function getAliasNamespace($alias)
    {
        throw new \BadMethodCallException('Namespace aliases not supported.');
    }

    public function setContainer(Application $container)
    {
        $this->container = $container;
    }
}

$managerRegistry = new ManagerRegistry(null, array(), array('doctrine_orm.em'), null, null, $app['doctrine_orm.proxies_namespace']); doesn't work.

I use:
$managerRegistry = new ManagerRegistry(null, array(), array('doctrine_orm.em'), null, null, '\Doctrine\ORM\Proxy\Proxy');

replace $this->extend with $app->extend

and add $app here:

function ($extensions, $app)