嵌套表单错误“必须管理传递给选择字段的实体”

我已经坚持了几天，并尝试了我可以在网上找到的所有解决方案，但没有运气 .

我有一个学生链接到许多转移 . 每次转移都与Classe相关联 .

要创建表单，我使用以下类：StudentType，TransferType .

StudentType表单classe有一个TransferType类型的字段 . 代码如下 .

StudentType.php

<?php
// src/PS/PSchoolBundle/Form/StudentType.php

namespace PS\PSchoolBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilder;
use PS\PSchoolBundle\Entity\Responsible;
use PS\PSchoolBundle\Entity\AuthorizedPerson;
use PS\PSchoolBundle\Entity\Transfer;

class StudentType extends AbstractType
{
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder
            ->add('subNumber', 'text', array('required'  => false))
            ->add('subDate', 'date', array(
                        'required'  => false,
                        'widget' => 'single_text',
                        'format' => 'dd/MM/yyyy',
                        'attr' => array('class' => 'datepicker')
                    ))
            ->add('lastName', 'text')
            ->add('firstName', 'text')
            ->add('lastNameSecondLanguage', 'text', array('required'  => false))
            ->add('firstNameSecondLanguage', 'text', array('required'  => false))
            ->add('birthDate', 'date', array('required'  => false,
                        'widget' => 'single_text',
                        'format' => 'dd/MM/yyyy',
                        'attr' => array('class' => 'datepicker')
                    ))
            ->add('birthPlace', 'text', array('required'  => false))
            ->add('birthPlaceSecondLanguage', 'text', array('required'  => false))
            ->add('gender', 'choice', array('choices' => array('' => 'Veuillez choisir', 'm' => 'Garçon', 'f' => 'Fille')))
            ->add('nationality', 'text', array('required'  => false))
            ->add('homeLanguage', 'text', array('required'  => false))
            ->add('bloodType', 'text', array('required'  => false))
            ->add('familySituation', 'choice', array('required'  => false,
                                                            'choices' => array(
                                                            '' => 'Veuillez choisir',
                                                            'm' => 'Mariés',
                                                            'd' => 'Divorcés',
                                                            'w' => 'Veuve',
                                                            'a' => 'Autre')))
            ->add('schoolStatusBefore', 'choice', array('required'  => false, 'choices' => array('' => 'Veuillez choisir', 's' => 'Scolarisé', 'ns' => 'Non scolarisé')))
            ->add('remark', 'textarea', array('required'  => false))
            ->add('responsibles', 'collection', array('type' => new ResponsibleType,
                                                      'prototype' => true,
                                                      'allow_add' => true,
                                                      'allow_delete' => true))
            ->add('authorizedPersons', 'collection', array('type' => new AuthorizedPersonType,
                                                      'prototype' => true,
                                                      'allow_add' => true,
                                                      'allow_delete' => true))

            ->add('transfers', 'collection', array('type' => new TransferType,
                                                      'prototype' => true,
                                                      'allow_add' => true,
                                                      'allow_delete' => true));
    }

    public function getName()
    {
        return 'student';
    }

    public function getDefaultOptions(array $options)
    {
        return array(
            'data_class' => 'PS\PSchoolBundle\Entity\Student'
        );
    }
}

TransferType.php

<?php
// src/PS/PSchoolBundle/Form/TransferType.php

namespace PS\PSchoolBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilder;
use PS\PSchoolBundle\Entity\Transfer;
use Doctrine\ORM\EntityRepository;

class TransferType extends AbstractType
{

    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder
            ->add('reason', 'choice', array(
                        'choices' => array('success' => 'Réussite', 'failure' => 'Échec'),
                        'required'  => false
                        ))
            ->add('date', 'date', array(
                        'widget' => 'single_text',
                        'format' => 'dd/MM/yyyy',
                        'attr' => array('class' => 'datepicker')
                    ))
            ->add('school', 'text', array('required'  => false))
            ->add('classe', 'entity', array('class' => 'PSPSchoolBundle:Classe', 'property' => 'name'));
    }

    public function getName()
    {
        return 'transfer';
    }

    public function getDefaultOptions(array $options)
    {
        return array(
            'data_class' => 'PS\PSchoolBundle\Entity\Transfer'
        );
    }
}

当我将“classe”字段添加到TransferType时，而不是显示表单，我抛出了这个错误：

必须管理传递到选择字段的实体500内部服务器错误 - FormException

请注意，我已经尝试过TransferType，并且填写了classe选择字段时表单正确显示 . 只有当我一起使用TransferType和StudentType时才会出现此问题 .

谢谢

编辑 - 以下是生成表单的控制器的操作：

public function byIdEditAction($id)
{
        $em = $this->getDoctrine()->getEntityManager();

        if(!$student = $em->getRepository('PSPSchoolBundle:Student')->find($id))
        {
        throw $this->createNotFoundException('Student[id='.$id.'] does not exist.');
        }

        $form        = $this->createForm(new StudentType(), $student);
        $formHandler = new StudentHandler($form, $this->get('request'), $em);

        if($formHandler->process())
        {
            $this->get('session')->setFlash('wehappy', "L'élève a été mis à jour.");
            return $this->redirect($this->generateUrl('_studentsbyidshow', array('id' => $student->getId())));
        }

        return $this->render('PSPSchoolBundle:Student:edit.html.twig', array(
            'form' => $form->createView(),
            'student' =>  $student
    ));
}