之前上传的图像在Sonata Admin中的记录更新中重新定位

Question

我在编辑表单上使用sonata admin bundle有一个奇怪的行为。我有一个名为工作表的字段，它接受PDF上传。文件上传正常，但只要对字段进行更新，该字段就会变空。我该如何防止这种行为？

我创建了下面的监听器，根据最新的symfony cookbook上传文件：http://symfony.com/doc/current/controller/upload_file.html

namespace AppBundle\EventListener;

use Symfony\Component\HttpFoundation\File\UploadedFile;
use Doctrine\ORM\Event\LifecycleEventArgs;
use Doctrine\ORM\Event\PreUpdateEventArgs;
use AppBundle\Entity\UNOE;
use AppBundle\Entity\COE;
use AppBundle\FileUploader;

class SheetUploadListener
{
    private $uploader;

    public function __construct(FileUploader $uploader)
    {
        $this->uploader = $uploader;
    }

    public function prePersist(LifecycleEventArgs $args)
    {
        $entity = $args->getEntity();

        $this->uploadFile($entity);
    }

    public function preUpdate(PreUpdateEventArgs $args)
    {
        $entity = $args->getEntity();

        $this->uploadFile($entity);
    }

    private function uploadFile($entity)
    {
        // upload only works for InspectionSheet entities
        if($entity instanceof UNOE) {

          $file = $entity->getWorksheet();
          // only upload new files
          if (!$file instanceof UploadedFile) {
              return;
          }
          $fileName = $this->uploader->upload($file);
          $entity->setWorksheet($fileName);
        }
        else {
          return;
        }

    }

    // public function postLoad(LifecycleEventArgs $args)
    // {
    //     $entity = $args->getEntity();
    //
    //     $fileName = $entity->getSheet();
    //
    //     $entity->setSheet(new File($this->targetPath.'/'.$fileName));
    // }
}

该字段包含在奏鸣曲管理员Admin中，如下所示：

protected function configureFormFields(FormMapper $formMapper)
{
    $formMapper->add('worksheet',FileType::class, array('label' => 'Inspection Worksheet', 'data_class' => null, 'required' => false))
}