Symfony 4 form with one-to-many relationship as checkbox

In symfony 4 i have a form to manage user details.

There is a one-to-many relationship between User class and SecurityGroup class.

In my form i would like to have all SecurityGroup listed with checkboxes.
I've managed to do it, but if i check a checkbox the group is not added to the user after the handle request. (In the request dump i see my 2 checked groups)

Form Field:

->add('SecurityGroups', EntityType::class, [
                'class'     => 'App\Entity\SecurityGroup',
                'choice_label' => 'displayname',
                'query_builder' => function (SecurityGroupRepository $repo) {
                    return $repo->createQueryBuilder('f')
                        ->where('f.id > :id')
                        ->setParameter('id', 1);
                },
                'label'     => 'Member Of?',
                'expanded'  => true,
                'multiple'  => true,
            ])

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Controller

public function edit(Request $request, $id)
    {

        $user = new User();

        if ($id == -1) {
            $module_title = 'Create a new User';
        }
        else
        {
            $module_title = 'Edit User';
            $user = $this->getDoctrine()->getRepository("App:User")->find($id);
        }

        $form = $this->createForm("App\Form\UserType", $user);


        $form->handleRequest($request);

        $current_user = $this->getUser();
        dump($request);
        dump($form);

        if ($form->isSubmitted() && $form->isValid())
        {

            $user = $form->getData();
            dump($user);
            
            $em = $this->getDoctrine()->getManager();
            $em->persist($user);
            $em->flush();
            $this->redirectToRoute('admin_user');
        }



        return $this->render('user/edit.html.twig', ['edit_form' => $form->createView(), 'module_title' => $module_title]);
    }

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Dumps

Request {#86 ▼
  +attributes: ParameterBag {#70 ▶}
  +request: ParameterBag {#69 ▼
    #parameters: array:1 [▼
      "user" => array:8 [▼
        "username" => "admin"
        "email" => "email@email.email"
        "password" => ""
        "isActive" => "1"
        "DisplayName" => "admin"
        "SecurityGroups" => array:2 [▼
          0 => "2"
          1 => "4"
        ]
        "save" => ""
        "_token" => "YaqXjd73ZhrX8p1eOiFWZi1JWivaCLKMGZbulTsiRgE"
      ]
    ]
  }

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