如何用Thymeleaf，JPA和Spring用另一个Select填充一个Select Box

Question

正如标题所述，我想知道如何在该项目中实现该功能。我是这个春季和Thymeleaf主题的新手，它使用CrudRepository，实体，每个存储库的服务及其控制器。

在简历中，我要通过国家选择框的选定值填充地区选择框，并保留第一个国家选择框的选定值。

以下是代码段：

LocationController.java：

@Controller
public class LocationController {
@Autowired
private NationService nationService;
@Autowired
private DistrictService districtService;
@GetMapping("/location")
public String init(Model model){
    model.addAttribute("nations", nationService.findAll());
    return "location";
}

@PostMapping("/setDistricts")
public String setDistricts(Model model, @RequestParam long id){
    model.addAttribute("districts", districtService.findAllByNation(id));
    return "location";
};
}

Nation.java ：（实体）

@Entity
public class Nation {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private long id;
@Column(name = "nation")
private String nation;

public long getId() {
    return id;
}

public void setId(long id) {
    this.id = id;
}

public String getNation() {
    return nation;
}

public void setNation(String nation) {
    this.nation = nation;
}
}

District.java ：（实体）

@Entity
public class District {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private long id;
@Column(name = "district")
private String district;
@ManyToOne
private Nation nation;

public long getId() {
    return id;
}

public void setId(long id) {
    this.id = id;
}

public String getDistrict() {
    return district;
}

public void setDistrict(String district) {
    this.district = district;
}

public Nation getNation() {
    return nation;
}

public void setNation(Nation nation) {
    this.nation = nation;
}
}

location.html：

<div class="form-group">
                <label for="nation"></label>
                <select class="custom-select" id="nation">
                    <th:block th:each="nation: ${nations}" th:onchange="@{/setDistricts(id=${nation.id})}">
                            <option th:value="${nation.id}" th:text="${nation.nation}"></option>
                    </th:block>
                </select>
            </div>

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