Form with Form Collection....

collection.php

<?php

namespace Test\VehicleBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class LicensesFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('year', 'number', array('required' => false))
            ->add('make', 'text', array('required' => false))
            ->add('model', 'text', array('required' => false))
            ->add('license', 'text', array('required' => false));

    }

    /**
     * @param OptionsResolver $resolver
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'Test\VehicleBundle\Entity\Vehicle'
        ));
    }

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

}

<?php

namespace Test\VehicleBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class LicenseFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder

            ->add('licenses', 'collection',
                array('type' => new LicensesFormType(),
                    'options' => array('required' => false)
                ));

    }

    /**
     * @param OptionsResolver $resolver
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'Test\VehicleBundle\Entity\Vehicle'
        ));
    }

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

}

/**
     * Displays all license plates for a vehicle entity.
     *
     * @Route("/licenses")
     * @Breadcrumb("Update License Plates")
     * @Template
     * @Method({"GET", "POST"})
     */
    public function licensesAction(Request $request)
    {
        $vehicles = $this->getDoctrine()->getRepository('VehicleBundle:Vehicle')->findAll();

        $licenseCollection = new Vehicle();

        foreach ($vehicles as $vehicle) {

            $licenseCollection->getLicenses()->add($vehicle);

        }

        $collection = $this->createForm(new LicenseFormType(), $licenseCollection);

        $collection->handleRequest($request);

        if ($collection->isValid()) {
            $em = $this->getDoctrine()->getManager();
            $em->persist($licenseCollection);

            $em->flush();

            $session = $this->getRequest()->getSession();
            $session->getFlashBag()->add('success', 'License Plates updated!');
        }

        return array(
            'forms' => $collection->createView()
        );
    }