@Test
public void testGetCityIdList() {

    // when empty then
    assertTrue(model.getCityIdList().isEmpty());

    // when
    model.addCity(new CityModel());
    // then
    assertEquals(1, model.getCityIdList().size());
    assertNull(model.getCityIdList().get(0));

    // when
    for (CityModel city : cities) {
        model.addCity(city);
    }

    // then
    assertEquals(CITIES_NUM + 1, model.getCityIdList().size());
    assertNull(model.getCityIdList().get(0));

    for (int i=0; i<cities.length; i++) {
        assertEquals(cities[i].id, model.getCityIdList().get(i+1));
    }
}