public void removeAllowedGroup(Long groupId) {
   UserGroup group = em.find(UserGroup.class, groupId);
   vdc.getAllowedGroups().remove(group);
   group.getVdcs().remove(vdc);
 }
 public void addAllowedGroup(Long groupId) {
   UserGroup group = em.find(UserGroup.class, groupId);
   vdc.getAllowedGroups().add(group);
   group.getVdcs().add(vdc);
 }