private Map<String, List<Attendance>> collectAttendancesByUsers(List<Attendance> attendances) {
        //comparing with login - because Employee class does not have own equals/hashcode
        Map<String, List<Attendance>> employeeMap = new HashMap<>();
        for (Attendance att : attendances) {
            String login = att.getEmployee().getLogin();
            List<Attendance> employeeAttendances = employeeMap.get(login);
            if (employeeAttendances == null) {
                employeeAttendances = new ArrayList<>();
                employeeMap.put(login, employeeAttendances);
            }
            employeeAttendances.add(att);
        }
        return employeeMap;
    }