Spring MVC @ModelAttribute bind subclasses

Question

I have 2 entity classes named District and Street, in View, to form is assigned Street object. How can I bind value of Street.district.id ? On for submitting I get NullPointerException .

View entities and form:

 <form class="form-inline" action="#" th:action="@{/direct/api/v1/newStreet}" th:object="${street}" method="POST">
            <div class="form-group">
                <label for="name">Name</label>
                <input type="text" th:field="*{name}" class="form-control" id="name" placeholder="Street name"/>
            </div>
            <div class="form-group">
                <label for="district">District</label>
                <select class="form-control" id="district">
                    <option th:each="d : ${districts}" th:field="*{district.id}" th:value="${d.id}" th:text="${d.name}"></option>
                </select>
            </div>
            <button type="submit" class="btn btn-default">Save</button>

Controller:

 @RequestMapping(value = "/newStreet", method = RequestMethod.POST)
 public String newStreet(@ModelAttribute Street street){
    log.info(street.getName());
    log.info(String.valueOf(street.getDistrict().getId()));
    return "redirect:/streets";
}

On form submitting I get Street name from input and null District.

Answer 1

Your error is in the select.

The attribute

th:field="*{district.id}"

should be in the select not in the option tag

Result:

 <select class="form-control" id="district" th:field="*{district.id}">
                    <option th:each="d : ${districts}"  th:value="${d.id}" th:text="${d.name}"></option>
                </select>