无法将现有实体映射到symfony2中的嵌入式表单

我有一个实体，它位于三个一对一映射的反面。实体FittingStep由映射FittingStepSingleValue，等等。它FittingStep具有一个字段fittingStepType，该字段标识FittingStep应寻找的三个实体之一。我想将该对象嵌入FittingStep编辑表单中。

我已经将表单定义为每个子表单的服务：

services:
    ihear.form.fitting_step_single_value:
        class: Ihear\FittingBundle\Form\FittingStepSingleValueType
        arguments: [@security.context]
        tags:
            -   
                name: form.type
                alias: ihear_fittingbundle_fittingstepsinglevaluetype
    ihear.form.fitting_step_double_value:
        class: Ihear\FittingBundle\Form\FittingStepDoubleValueType
        arguments: [@security.context]
        tags:
            -   
                name: form.type
                alias: ihear_fittingbundle_fittingstepdoublevaluetype
    ihear.form.fitting_step_option:
        class: Ihear\FittingBundle\Form\FittingStepOptionType
        arguments: [@security.context]
        tags:
            -   
                name: form.type
                alias: ihear_fittingbundle_fittingstepoptiontype

这些服务类看起来像这样（非常基础）

class FittingStepSingleValueType extends AbstractType
{
    private $securityContext;

    public function __construct(SecurityContext $securityContext)
    {   
        $this->securityContext = $securityContext;
    }   

    public function buildForm(FormBuilderInterface $builder, array $options)
    {   
        $builder
            ->add('max1')
            ->add('description1')
            ->add('fittingStep', 'hidden')
        ;   
    }   

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'Ihear\FittingBundle\Entity\FittingStepSingleValue'
        ));
    }

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

我的表单在PRE_SET_DATA上使用EventListener来添加适当的嵌入式表单字段：

class FittingStepType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {   
        $builder
            ->add('name')
            ->add('abbreviation')
            ->add('description')
            ->add('fittingStepType',
                'choice',
                ['choices' => ['SingleValue' => 'SingleValue',
                               'DoubleValue' => 'DoubleValue',
                               'Option' => 'Option'],
                 'empty_value' => 'select one please'])
        ;   

        $formModifier = function(FormInterface $form, $fittingStepType) {
            switch ($fittingStepType) {
                case 'SingleValue':
                    $form->add('fittingStepSingleValue',
                        'ihear_fittingbundle_fittingstepsinglevaluetype');
                    if ($form->has('fittingStepDoubleValue'))
                        $form->remove('fittingStepDoubleValue');
                    if ($form->has('fittingStepOption'))
                        $form->remove('fittingStepOption');
                    break;
                case 'DoubleValue':
                    $form->add('fittingStepDoubleValue',
                        'ihear_fittingbundle_fittingstepdoublevaluetype');
                    if ($form->has('fittingStepSingleValue'))
                        $form->remove('fittingStepSingleValue');
                    if ($form->has('fittingStepOption'))
                        $form->remove('fittingStepOption');
                    break;
                case 'Option':
                    $form->add('fittingStepOption',
                        'ihear_fittingbundle_fittingstepoptiontype');
                    if ($form->has('fittingStepSingleValue'))
                        $form->remove('fittingStepSingleValue');
                    if ($form->has('fittingStepDoubleValue'))
                        $form->remove('fittingStepDoubleValue');
                    break;
            }
        };

        $builder->addEventListener(
            FormEvents::PRE_SET_DATA,
            function(FormEvent $event) use ($formModifier) {
                $form = $event->getForm();
                // this is the FittingStep
                $data = $event->getData();
                // this is the Entity that contains the value(s)
                // i.e. FittingStepSingleValue
                $fittingStepType = $data->getFittingStepType();
                switch ($fittingStepType) {
                    case 'SingleValue':
                        $formModifier($form, $fittingStepType);
                        break;
                    case 'DoubleValue':
                        $formModifier($form, $fittingStepType);
                        break;
                    case 'Option':
                        $formModifier($form, $fittingStepType);
                        break;
                }
            }
        );
    }
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'Ihear\FittingBundle\Entity\FittingStep'
        ));
    }

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

我的编辑操作的控制器：

/** 
 * Displays a form to edit an existing FittingStep entity.
 *
 * @Route("/{id}/edit", name="admin_fittingstep_edit")
 * @Method("GET")
 * @Template()
 */
public function editAction($id)
{   
    $em = $this->getDoctrine()->getManager();

    $entity = $em->getRepository('IhearFittingBundle:FittingStep')->find($id);

    if (!$entity) {
        throw $this->createNotFoundException(
            'Unable to find FittingStep entity.');
    }   

    $editForm = $this->createForm(new FittingStepType(), $entity);
    $deleteForm = $this->createDeleteForm($id);

    return array(
        'entity'      => $entity,
        'edit_form'   => $editForm->createView(),
        'delete_form' => $deleteForm->createView(),
    );  
}   

当我尝试加载表单时，出现异常：

表单的视图数据应为标量类型，数组类型或\ ArrayAccess实例，但它是Ihear \ FittingBundle \ Entity \ FittingStep类的实例。您可以通过将“ data_class”选项设置为“ Ihear \ FittingBundle \ Entity \ FittingStep”或添加一个将类Ihear \ FittingBundle \ Entity \ FittingStep的实例转换为标量，数组或\实例的视图转换器来避免此错误。 ArrayAccess。

我究竟做错了什么？实体之间的对象映射与表单上的映射之间似乎有些脱节。附带说明一下，我$formModifier在创建表单中成功使用了闭包，因此在使用嵌入式表单创建新实体时，闭包可以正常工作。