Php Symfony表单提交后提交填充的实体类型

我想用这种结构制作一个“两级”动态符号窗体：

技能卡：EntityType

会话：EntityType

回合：选择类型

选择技能卡时，我填充会话字段，选择会话后，我填充回合字段

就在这个时候，我做了这个

$builder->add('skillCard', EntityType::class, [
        'class' => SkillCard::class,
        'choices' => $student->getSkillCards()->filter(function (SkillCard $skillCard) {
            return $skillCard->getStatus() != EnumSkillCard::EXPIRED &&
                count($skillCard->getSkillCardModulesNotPassed()) > 0;
        }),
        'placeholder' => ''
    ]);
$builder->addEventListener(
        FormEvents::PRE_SET_DATA,
        function (FormEvent $formEvent) {
            /** @var Booking */
            $data = $formEvent->getData();
            $this->addSessionField($formEvent->getForm(), $data->getSkillCard());
            $this->formModifierBySession($formEvent->getForm(), $data->getSession());
        }
    );
$builder->get('skillCard')->addEventListener(
        FormEvents::POST_SUBMIT,
        function (FormEvent $formEvent) {
            $skillCard = $formEvent->getForm()->getData();
            $this->addSessionField($formEvent->getForm()->getParent(), $skillCard);
        }
    );

protected function addSessionField(FormInterface $form, SkillCard $skillCard = null)
{
    $builder = $form->getConfig()->getFormFactory()->createNamedBuilder('session', EntityType::class, null, [
        'auto_initialize' => false,
        'class' => Session::class,
        'placeholder' => '',
        'query_builder' => function (EntityRepository $er) use ($skillCard) {
            $qb = $er->createQueryBuilder('s');
            if (is_null($skillCard)) {
                $qb->where('s = 0');
            } else {
                $qb->where('s.certification = :id')
                    ->andWhere('s.subscribeExpireDate > :date')
                    ->setParameter('id', $skillCard->getCertification())
                    ->setParameter('date', new DateTime());
            }
            return $qb;
        },
    ]);

    $builder->addEventListener(
        FormEvents::POST_SUBMIT,
        function (FormEvent $formEvent) {
            $session = $formEvent->getForm()->getData();
            $this->formModifierBySession($formEvent->getForm()->getParent(), $session);
        }
    );
    $form->add($builder->getForm());
}

protected function formModifierBySession(FormInterface $form, Session $session = null)
{
    $form->add('turn', ChoiceType::class, [
        'choices' => is_null($session) ? [] : range(0, $session->getRounds()),
    ]);
}

这是javascript代码（用于ajax调用）：

当我选择skillCard时，没有问题，但是当我选择会话时，我得到一个TransformationFailedException:“无法反转属性路径“会话”的值：选择“x”不存在或不唯一”，因此不会填充turn字段，因为

$session=$formEvent->getForm（）->getData（）为空

任何建议都将不胜感激

$(document).ready(function () {
    const $skillCard = $('#booking_skillCard');
    const $form = $('form[name = "booking"]');

    $skillCard.change(function () {
        sendAjax($skillCard, $form, function (html) {
            replaceFormField('#booking_module', html);
            replaceFormField('#booking_session', html);
            sessionField($form);
        });
    });
});

function sessionField($form) {
    const $session = $('#booking_session');
    $session.change(function () {
        sendAjax($session, $form, function (html) {
            replaceFormField('#booking_turn', html);
        });
    });
}

function sendAjax($jObj, $form, successCallBack) {
    let data = {};
    data[$jObj.attr('name')] = $jObj.val();
    console.log($form);
    $.ajax({
        url: $form.prop('action'),
        type: $form.prop('method'),
        data: data,
        success: successCallBack
    })
}

function replaceFormField(selector, html) {
    $(selector).replaceWith(
        $(html).find(selector)
    );
}