    @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));
        }
    }