Room.php

/**
 * @ORM\Table(name="room")
 * @ORM\Entity(repositoryClass="Ahsio\StackBundle\Repository\RoomRepository")
 */
class Room
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;
    
    /**
     * @ORM\Column(type="string", length="255", name="name")
     */
    protected $name;

    /**
     * @ORM\Column(type="text", name="description")
     */
    protected $description;
    
    /**
     * @ORM\OneToMany(targetEntity="Ahsio\StackBundle\Entity\Workstation", mappedBy="room", cascade={"persist", "remove"}, orphanRemoval=true)
     */
    protected $workstations;

RoomType.php

     * (non-PHPdoc)
     * @see Symfony\Component\Form.AbstractType::buildForm()
     */
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder
            ->add('name', 'text', array(
                    'label' => 'toto'))
            ->add('description')
            ->add('workstations', 'entity', array(
                'multiple'    => true,
                'expanded' => true,
                'property'		=> 'number',
                'class'         => 'Ahsio\StackBundle\Entity\Workstation',
                'query_builder' => function(\Ahsio\StackBundle\Repository\WorkstationRepository $wr) {
                    return $wr->createQueryBuilder('w')
                    ->orderBy('w.number', 'ASC'); },
            ))
        ;
    }

    /**
     * (non-PHPdoc)
     * @see Symfony\Component\Form.AbstractType::getDefaultOptions()
     */
    public function getDefaultOptions(array $options)
    {
        return array(
            'data_class' => 'Ahsio\StackBundle\Entity\Room',
        );
    }

template.html.twig

<form method="post" action="{{ path('stack_room_create') }}" {{ form_enctype(form) }} >
    {{ form_row(form.name) }}
    {{ form_row(form.description) }}
    {% for field in form.workstations %}
       <li>
         {{ form_widget(field) }}
         {{ form_label(field) }}
       </li>
    {% endfor %}
    {{ form_rest(form) }}
    <input type="submit" value="save"/>
</form>

Workstation.php

/**
 * @ORM\Table(name="workstation")
 * @ORM\Entity(repositoryClass="Ahsio\StackBundle\Repository\WorkstationRepository")
 */
class Workstation
{

    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @ORM\ManyToOne(targetEntity="Ahsio\StackBundle\Entity\Room")
     * @ORM\JoinColumn(name="room_id", referencedColumnName="id", nullable=false)
     */
    private $room;

    /**
     * @ORM\Column(name="number", type="integer", nullable=true)
     */
    private $number;