Symfony3 Ajax Form Add Extra Field

AdminBundle\Form\CoreForm.php:

class CoreForm extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        /**
        Ajax Call : onSubmitAdd
        */
        $builder->add('set_core_doctrine_metadata_type', ChoiceType::class, array(
            'label' => 'Doctrine Metadata Cache Driver',
            'choices' => $this->getDoctrineAccelerator(),
            'empty_data' => 'array',
            'choice_translation_domain' => false,
            'attr' => array(
                'onSubmitAdd' => 'set_core_doctrine_metadata_host|set_core_doctrine_metadata_port'
            )
        ))

        /**
        Ajax Response : onSubmitAdd
        */
        $doctrineModify = function (FormInterface $form){
            $metaDataDriver = $form->get('set_core_doctrine_metadata_type')->getData();

            if (!$form->isValid()) {
                if ($metaDataDriver == 'memcache' || $metaDataDriver == 'memcached'){
                    $form->add('set_core_doctrine_metadata_host', TextType::class, array(
                        'label' => 'Doctrine Metadata Cache Host',
                    ))
                    ->add('set_core_doctrine_metadata_port', IntegerType::class, array(
                        'label' => 'Doctrine Metadata Cache Port',
                    ));
                }
            }
        };

        $builder->addEventListener(FormEvents::SUBMIT, function (FormEvent $event) use ($doctrineModify){
            $form = $event->getForm();
            $doctrineModify($form);
        });
    }
}

Form Type: Form working smoothly. But it does not allow new field.

jQuery Ajax Code:

$(document).on('change', '[onSubmitAdd]', function () {
    // Get Form
    form = $(this).closest('form');

    // Send Data
    var data = {};
    data[$(this).attr('name')] = $(this).val();

    // Html Block
    formGroup = $(this).closest('.form-group');
    getFormGroup = $(this).attr('onSubmitAdd');
    getFormGroup = getFormGroup.split('|');

    $.ajax({
        url: form.attr('action'),
        type: form.attr('method'),
        data: data,
        success: function (data) {
            addContent = "";

            // Get Items
            $.each(getFormGroup, function (index, item) {
                dt = $(data.content.body).find('.' + item).html();
                if (typeof dt != 'undefined'){
                    addContent = addContent + '<div class="form-group">'+ dt +'</div>';
                }
            });

            // Replace or Add
            if (formGroup.next().hasClass('onSubmitAdded')){
                formGroup.next().html(addContent);
            } else {
                formGroup.after('<div class="onSubmitAdded">'+ addContent +'</div>');
            }
        }
    });
});

Problem: Ajax system is working smoothly. But when the form is submitted, the error code "This form should not contain extra fields"

To be able to accept the extra submitted choices, you have to add them in your form on the PRE_SUBMIT Event :

 class FooType extends AbstractType
 {
     public function buildForm(FormBuilderInterface $builder, array $options)
     {
         $builder
             ->add('tag', ChoiceType::class, array('choices'=>array()))
         ;

         $builder->addEventListener(
             FormEvents::PRE_SUBMIT,
             function(FormEvent $event){
                 // Get the parent form
                 $form = $event->getForm();

                 // Get the data for the choice field
                 $data = $event->getData()['tag'];

                 // Collect the new choices
                 $choices = array();

                 if(is_array($data)){
                     foreach($data as $choice){
                         $choices[$choice] = $choice;
                     }
                 }
                 else{
                     $choices[$data] = $data;
                 }

                 // Add the field again, with the new choices :
                 $form->add('tag', ChoiceType::class, array('choices'=>$choices));
             }
         );
     }
 }

To avoid "Notice: Array to string conversion" Think to copy all field array parameters in new field in $form, for example, if you have 'multiple' => true in $builder 'tag' put it in $form 'tag' too