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);
}