我们可以使用Symfony表单组件中的Data Transformers来解决此问题，将从表单中收集的数据转换为对应的实体对象。以下是一个例子：

use Symfony\Component\Form\DataTransformerInterface;

class EntityToIdTransformer implements DataTransformerInterface
{
    private $entityManager;
    private $entityClass;

    public function __construct(EntityManagerInterface $entityManager, string $entityClass)
    {
        $this->entityManager = $entityManager;
        $this->entityClass = $entityClass;
    }

    public function transform($value)
    {
        if (null === $value) {
            return '';
        }

        return $value->getId();
    }

    public function reverseTransform($id)
    {
        if (!$id) {
            return null;
        }

        return $this->entityManager->getRepository($this->entityClass)->find($id);
    }
}

此转换器将一个实体转换为其ID，以便在表单中进行显示，然后通过ID将其转换回实体。我们可以将其与表单中的字段关联：

$builder->add('user', EntityType::class, [
    //...
    'choice_label' => function ($user) {
        return $user->getFullName();
    },
    'class' => User::class,
    'required' => true,
    'attr' => [
        'class' => 'select2'
    ],
    'query_builder' => function (EntityRepository $er) {
        return $er->createQueryBuilder('u')
            ->orderBy('u.fullName', 'ASC');
    ],
    'choice_attr' => function ($user) {
        return [
            'data-email' => $user->getEmail(),
            'data-phone' => $user->getPhone(),
        ];
    },
    'choice_value' => 'id',
    'choice_loader' => function ($entityClass) use ($entityManager) {
        return new EntityLoader($entityManager, $entityClass);
    },
    'choice_translation_domain' => false,
    'data_class' => null,
    'empty_data' => '',
    'empty_value' => '',
    'error_bubbling' => true,
    'invalid_message' => '',