Dto中的嵌套对象为NULL

Question

大家好！ 这是我的第一篇文章，请谅解。我在github的第一个应用程序中的代码有问题。我有视图，尝试在数据库中添加带有答案的问题。两种模型都以一个视图的形式创建。发布表单时，我得到这样的对象：

NewQuestionDto (
question = QuestionDto (
id=null, 
question=zzz, 
points=12, 
difficultyLevel=ADVANCED, 
createdAt=2018-08-24, 
categoryDto = CategoryDto (
id=1, 
name=null
), 
authorDto = UserDto (
id=1, 
name=null, 
surname=null, 
age=null, 
email=null, 
gender=null, 
points=0,
role=USER, 
createdAt=2018-08-24
)
), 
answers=[
AnswerDto(id=null, answer=q, correctAnswer=false, questionDto=null),         AnswerDto(id=null, answer=w, correctAnswer=true, questionDto=null), AnswerDto(id=null, answer=e, correctAnswer=false, questionDto=null), AnswerDto(id=null, answer=r, correctAnswer=false, questionDto=null)
])

我不知道为什么answerDto中的questionDto对象为空？

MyController：

@GetMapping("/question/add")
public String addQuestionGet(Model model){
    NewQuestionDto newQuestionDto = NewQuestionDto.builder()
            .question(new QuestionDto())
            .answers(Arrays.asList(
                    new AnswerDto(),
                    new AnswerDto(),
                    new AnswerDto(),
                    new AnswerDto()
             ))
            .build();
    model.addAttribute("newQuestion", newQuestionDto);
    model.addAttribute("difficultyLevels", DifficultyLevel.values());
    model.addAttribute("categories", categoryService.findAllCategories());
    model.addAttribute("authors", userService.findAllUsersWithAdminRole());
    return "/dashboard/question/add";
}

@PostMapping("/question/add")
public String addQuestionPost(@ModelAttribute NewQuestionDto newQuestionDto){
    System.out.println(newQuestionDto);
    questionService.addOrUpdateQuestion(newQuestionDto.getQuestion());
    for (AnswerDto a : newQuestionDto.getAnswers()){
        answerService.addOrUpdateAnswer(a);
    }
    return "redirect:/dashboard/questions/all";
}

NewQuestionDto

public class NewQuestionDto {
private QuestionDto question;
private List<AnswerDto> answers;

}

我的HTML文件：

<form method="POST" th:action="@{/dashboard/question/add}" th:object="${newQuestion}">
                <!--Some code here-->
                <h4 class="muted mt-4">Answers</h4>
                <hr>
                <div class="form-group mt-3">
                    <div class="form-inline mt-1" th:each="a, stat : *{answers}">
                        <input class="form-control mr-2" type="text" th:placeholder="Answer + ' ' + ${stat.index}" th:field="*{answers[__${stat.index}__].answer}" autocomplete="off"/>
                        <input type="checkbox" th:field="*{answers[__${stat.index}__].correctAnswer}"/>
                    </div>
                </div>
                <!--Buttons-->
                <div class="form-group mt-3">
                    <button type="submit" class="btn btn-success btn-block">ADD</button>
                    <button type="reset" class="btn btn-outline-danger btn-block">RESET</button>
                </div>
            </form>

表数据库中的“答案”：

如果您需要我的应用程序中的更多代码，请告诉我。

谢谢！

已编辑

AnswerServiceImpl

@Override
public void addOrUpdateAnswer(AnswerDto answerDto) {
    answerRepository.addOrUpdate(ConverterDto.fromAnswerDtoToAnswer(answerDto));
}

QuestionServiceImpl

@Override
public void addOrUpdateQuestion(QuestionDto questionDto) {
    questionRepository.addOrUpdate(ConverterDto.fromQuestionDtoToQuestion(questionDto));
}

ConverterDTO

public static Answer fromAnswerDtoToAnswer(AnswerDto answerDto){
    return answerDto == null ? null : new Answer(
            answerDto.getId(),
            answerDto.getAnswer(),
            answerDto.getCorrectAnswer(),
            fromQuestionDtoToQuestion(answerDto.getQuestionDto())
    );
}

public static Question fromQuestionDtoToQuestion(QuestionDto questionDto){
     return questionDto == null ? null : new Question(
            questionDto.getId(),
            questionDto.getQuestion(),
            questionDto.getPoints(),
            questionDto.getDifficultyLevel(),
            questionDto.getCreatedAt(),
            fromCategoryDtoToCategory(questionDto.getCategoryDto()),
            fromUserDtoToUser(questionDto.getAuthorDto()),
            new HashSet<>(),
            new HashSet<>()
    );
}