Dependent drop-down list from map in Thymeleaf

Question

I want to create a drop-down list with countries and the second drop-down list with cities, which depends on selected value in the first list. And the list of cities should be changed dynamically. In the view (Thymeleaf) I have a Map<CountryModel, Set<RegionModel>> from controller. CountryModel's name should be shows in the second drop-down list, and Set should be shows in the second(dependent) drop-down list.
Here I create first drop-down list:

 <tr>
   <td th:text="#{country}"/>
   <td>
      <div class="form-group">
          <select th:field="*{transferRequestModel.country}" class="form-control" id="country">
                <option th:each="country : ${transferModel.countries}"
                    th:value="${country}"
                    th:text="${country.key.countryName}">Wireframe
                </option>
          </select>
      </div>
    </td>
 </tr>

So how to create second drop-down list which depends on selected country in the first list?

Answer 1

So I have solved my problem with AJAX request and jQuery append.

1. Change Map<CountryModel, Set<RegionModel>> to Map<String, Set<String>>

2. AJAX request

   function sendAjaxRequest() {
       var country = $("#country").val();
       $.get( "/regions?country=" + country, function( data ) {
           $("#region").empty();
           data.forEach(function(item, i) {
               var option = "<option value = " + item + ">" + item +  "</option>";
               $("#region").append(option);
           });
       });
   };
   

3. Use sendAjaxRequest() when i change first drop-down list.

   $(document).ready(function() {
       $("#country").change(function() {
           sendAjaxRequest();
       });
   });
   

4. Drop-down list at the Thymeleaf template

First drop-down list

<td th:text="#{country}"/>
<td>
    <div class="form-group">
        <select th:field="*{model.country}" class="form-control" id="country">
            <option th:each="country : ${model.countries}"
                    th:value="${country}"
                    th:text="${country}">Wireframe
            </option>
        </select>
    </div>
</td>

Second drop-down list

<td>
    <div class="form-group">
        <select th:field="*{requestModel.region}" class="form-control" id="region">
        </select>
    </div>
</td>

5. Controller

   @RequestMapping(value = "/regions")
   @ResponseBody
   public Set getRegions(@RequestParam String country) {
       Map<String, Set<String>> regions = regionsService.getRegions();
       return regions.get(country);
   }