gistfile1.php

class Formation
{
  ...
    /**
     * @Serializer\Exclude 
     * @ORM\OneToMany(targetEntity="FormationRequired", mappedBy="receiver" , cascade={"persist", "remove"} , orphanRemoval=true)
     */
    private $formationRequireds;
    
    /**
     * Add formationRequireds
     *
     * @param \CIV\ApiBundle\Entity\FormationRequired $formationRequireds
     * @return Formation
     */
    public function addFormationRequired(\CIV\ApiBundle\Entity\FormationRequired $formationRequireds)
    {
        $this->formationRequireds[] = $formationRequireds;
        $formationRequireds->setReceiver($this);
        var_dump('addFormationRequired');
        
        return $this;
    }
}

class FormationType extends AbstractType
{
    /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
  
            ->add('formationRequireds', 'entity', array(
                'class' => 'Bundle:FormationRequired',
                'property' => 'id',
                'multiple' => true,
                'query_builder' => function(EntityRepository $er) {
                    return $er->createQueryBuilder('u')->orderBy('u.id', 'ASC');
                }
            ))
        ;
    }
  
}