Workaround to get the same feature as in https://github.com/symfony/symfony/pull/6910

CollectionTypeExtension.php

<?php

namespace Incenteev\Bundle\Form\Extension;

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

/**
 * @author Adrien Brault <[email protected]>
 */
class CollectionTypeExtension extends AbstractTypeExtension
{
    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        if (null !== ($prototypeFormBuilder = $builder->getAttribute('prototype'))) {
            /** @var $prototypeFormBuilder FormBuilderInterface */
            $prototypeFormBuilder->setData($options['prototype_data']);
        }
    }

    /**
     * {@inheritdoc}
     */
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'prototype_data' => null,
        ));
    }

    /**
     * {@inheritdoc}
     */
    public function getExtendedType()
    {
        return 'collection';
    }
}

Thanks so much for this :)

Thanks! Works perfectly!

Thanks