Symfony形式：嵌入式表单的错误错误映射

Question

我正在使用带有silex的Symfony2表单。

我创建了这样的表单：

$form = $app->form()
    ->add('foo', new FooType())
    // ->add(...)
    ->add('save', 'submit')
    ->getForm();

FooType：

<?php

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;
use Symfony\Component\Validator\Constraints as Assert;

class FooType extends AbstractType
{

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('bar', 'text', ['constraints' => new Assert\NotBlank()]);
        $builder->add('baz', 'text', ['required' => false]);
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults([
            'error_bubbling' => false,
        ]);
    }

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

如果“bar”字段为空，则错误始终映射到“foo”基本字段。为什么？如何将错误映射到字段本身（“bar”）？

编辑：

表单呈现方式如下（Twig）：

{{ form(form) }}

提交空字段后的结果：

<form name="form" method="post" action="">
    <div>
        <label class="required">Foo</label>
        <ul>
            <li>This value should not be blank.</li>
        </ul>
        <div id="form_foo">
            <div>
                <label for="form_foo_bar" class="required">Bar</label>
                <input type="text" id="form_foo_bar" name="form[foo][bar]" required="required">
            </div>
            <div>
                <label for="form_foo_baz">Baz</label>
                <input type="text" id="form_foo_baz" name="form[foo][baz]">
            </div>
        </div>
    </div>
    <div>
        <button type="submit" id="form_save" name="form[save]">Save</button>
    </div>
    <input type="hidden" id="form__token" name="form[_token]" value="faebf2a0b0a05ddb8e5f40d58c7a3bf3d49522d4">
</form>