Allowing extra keys in forms by removing them from the submitted data

ExtraKeysExtension.php

<?php

namespace Acme\DemoBundle\Form;

use Symfony\Component\Form\AbstractTypeExtension;
use Symfony\Component\Form\FormBuilderInterface;

class ExtraKeysExtension extends AbstractTypeExtension
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->addEventSubscriber(new ExtraKeysListener());
    }
    
    public function getExtendedType()
    {
        return 'form';
    }
}

ExtraKeysListener.php

<?php

namespace Acme\DemoBundle\Form;

use Symfony\Component\Form\FormEvents;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

class ExtraKeysListener implements EventSubscriberInterface
{
    public static function getSubscribedEvents()
    {
        // The priority must be lower than the event listener of the CollectionType to avoid breaking it
        return array(FormEvents::PRE_SUBMIT => array('cleanData', -5));
    }
    
    public function cleanData(FormEvent $event)
    {
        $form = $event->getForm();
        $data = $event->getData();
        
        if (!is_array($data)) {
            return;
        }
        
        $data = array_intersect_key($data, $form->all());

        $event->setData($data);
    }
}

services.xml

<?xml version="1.0" ?>

<container xmlns="http://symfony.com/schema/dic/services"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="acme_demo.form.extra_keys_extension" class="Acme\DemoBundle\Form\ExtraKeysExtension">
            <tag name="form.type_extension" alias="form" />
        </service>
    </services>

</container>