trinity-cms-bundles/src/Trinity/Bundle/UserBundle/Form/EventListener/AddCodeFieldSubscriber.php

<?php

namespace Trinity\Bundle\UserBundle\Form\EventListener;

use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\Validator\Constraints\NotBlank;

class AddCodeFieldSubscriber implements EventSubscriberInterface
{
    public static function getSubscribedEvents()
    {
        return array(FormEvents::PRE_SET_DATA => 'preSetData');
    }

    public function preSetData(FormEvent $event)
    {
        $data = $event->getData();
        $form = $event->getForm();

        if (!$data || !trim($data->getCode())) {
            $form->add(
                'code',
                'text',
                array(
                    'required' => true,
                    'constraints' => array(
                        new NotBlank(),
                    )
                )
            );
        }
    }
}