Form Builder call in a form a form collection without "direct liaison"

Question

In a form Demand, I want to create an Article, but Article and Demand are not directly joined tables, so how can I do that?

My Database conception:

| demand   |  1,n       |  listing |  n,n | article |

(Sorry, I can upload an image)

I'm lost; should I call a service or something?
I'd like to follow best practice in doing this.

Answer 1

I have found a solution. For make that i have simply call form to create Article and Demand, from the form for create listing.

Like that

<?php

namespace OrderIT\Bundle\OrderBundle\Form;

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

class ListingType extends AbstractType
{
    /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('demandDemand', new DemandType())
            ->add('articleArticle', 'collection', array(
               'type' => new ArticleType(),
               'allow_add' => true,
               'by_reference' => false,))
        ;
    }

    /**
     * @param OptionsResolverInterface $resolver
     */
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'OrderIT\Bundle\OrderBundle\Entity\Listing'
        ));
    }

    /**
     * @return string
     */
    public function getName()
    {
        return 'orderit_bundle_orderbundle_listing';
    }
}