Spring Controller Test - object in controller is null

Question

I try to test a controller and have a problem with an object City c = cityService.findByID(city); It is always null when I run the test. If I run the program and not the test it works as it should.

City c shouldn't be null because it is an object which should be fetched from DB by city id of last selected city(option from dropdown list) and I pass the city id here: .param("city", String.valueOf(306)).

void selectCityTest() throws Exception { //post
    Integer voivodeship = 10;
    Integer city = 306;

    List<City> cities = cityService.getAllCitiesByVoivodeship(voivodeship);
    mockMvc.perform(MockMvcRequestBuilders.post("/select_city")
        .contentType(MediaType.APPLICATION_JSON)
        .param("city", String.valueOf(306))
        .content(new Gson().toJson(cities)))
        .andExpect(status().isOk());
}

Controller.java

@RequestMapping(value = "/select_city", method = RequestMethod.POST)
public String selectCity(Integer city, Model model) {

    City c = cityService.findByID(city);
    List<City> cities = cityService.getAllCitiesByVoivodeship(c.getVoivodeship().getId());
    model.addAttribute("cities", cities);
    model.addAttribute("city_selected", city);

    return "taxoffice";
}