@Override
public List<Person> findByGroup(String groupId) {
TypedQuery<Group> query = manager.createNamedQuery(Group.FIND_BY_TITLE, Group.class);
query.setParameter(Group.GROUP_ID_PARAM, groupId);
Group result = getSingleResult(query.getResultList());
return result == null ? new ArrayList<Person>() : result.getMembers();
}