如何仅更新symfony2中实体的某个字段

I have a user table. In that table I have different fields. I only need to update the image column of the user table.

I have a form for uploading the image only. Even though the image is set but when I submit the form I get validations errors of other fields.

I know that the validation errors are because I passed the user object to the form and issued a isValid().

How do I only update the specific column like the image column?

public function updateImageAction(Request $request){

        $form = $this->createForm(new UploadImageType(),new User(),[
                'action' => $this->generateUrl('update_image')
        ]);


        $form->handleRequest($request);
        $user = $this->getUser();
        if ($form->isSubmitted() && $form->isValid())
        {
            $em = $this->getDoctrine()->getManager();

            $data = $form->getData();

            dump($data);die;

            $em->persist($data);
            $em->flush();
        }

        return $this->render('photo.html.twig', [
                'form' => $form->createView()

        ]);
    }