Symfony2 form entity field different beahviour

Question

I'm using Symfony2 since few months. I'm able to accomplish basic operations such CRUD, etc. However I'm facing an issue now, regarding a two-entity form behaviour. I've got a unique form where there're 2 entities connected to 2 different tables:

• guests
• email suggested by guests

where every email_id is a foreign key into the guest row. Every guest may have no emails, than a null foreign key for that particular row.

Once the form has been sent by the user, it will be saved a new record into the guest table but I'd like that the field email (belonging to another entity) would be saved into a different table and performing some checks:

• if the email is present into the table it should't be saved but I need to take its id and save it as foreign key into the new guest row;
• if the email isn't into the table, then it has to be saved and connected to the new guest;
• if the guest doesn't fill any email, then a null foreign key has to be put against the guest and no record will be saved into the email table.

I found many articles about entity-form CRUD operations but few or nothing about how to customise insert/update at one time according to a specific field in Symfony2.

Please can someone explains me how to do it or if there's somebody who knows articles/examples for, please, can he reports to me those articles?

This is a snippet of the form:

class InYourMindType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('title', 'text', array(
            'label' => 'label_think_title',
            'required' => true,
            'attr' => array(
                'title' => 'label_think_title',
                'class' => 'form-control'
            )
        ))->add('thinkFriend',
                new InYourMindFriendType

where new InYourMindFriendTypeis the email. Below the controller's snippet instead. At the moment just a simple method which saves the form

function sendInYourMindAction(Request $request)
{
    $EnForm = new InYourMind;

    $form = $this->createForm(new InYourMindType, $EnForm);

    if ($request->getMethod() === 'POST') {
        $form->bind($request);

        if ($form->isValid()) {
            $em = $this->getDoctrine()
                    ->getEntityManager();
            $em->merge($EnForm);
            $em->flush();

            $this->get('session')
                    ->getFlashBag()
                    ->set('think-success-notice', 'message successfully sent');

            return $this->redirect($this->generateUrl('commonMainBundle_homepage'));
        }
    }


    return 'something';

}