如何从下拉框中获取用户选择的值并将其添加到模型中？ [重复]

Question

这个问题在这里已有答案：

How do I get the selected value from a drop-down menu with Thymeleaf? 1回答

我有一个用户正在创建新考试的表单，用户可以从下拉菜单中选择一个主题。此下拉列表包含主题字符串而非实际主题对象。在我的程序中，存在与考试具有一对多关系的实际主题对象。

如何找到用户选择的值？我想将它添加到模型中，以便我可以创建一个考试对象并将其添加到数据库中。

将用户选择的主题字符串添加到模型后，如何找到用户选择的值？因为我想将他们选择的内容设置为相同的主题名称，然后在主题库中搜索具有相同名称的主题，以便可以将考试添加到该主题。

希望我的代码能让这一点更加清晰。

我收到了这个错误。绑定结果和bean名称主题的普通目标对象都不可用作请求属性。

这个代码形成了我的控制器

@GetMapping("/addexam")
public String showExamForm(Model model) {

    // Here I am finding the subjects that belong to the current user 

  //and adding them as strings to an arraylist. 


  //I populate the dropdown with strings fromthis arraylist.        


    Authentication loggedInUser = SecurityContextHolder.getContext().getAuthentication();
    String email = loggedInUser.getName();   

    User user = userRepository.findByEmailAddress(email);

    ArrayList<String> subjects = new ArrayList<String>();

    for(Subject sub:user.getSubject())
    {
        subjects.add(sub.getSubjectName());
    }
        model.addAttribute("subjects", subjects);

return "addExam";
}

@PostMapping("/addexam")
public String addNewExam(@ModelAttribute("exam") @Valid @RequestBody Exam 
 exam,UserRegistrationDto userDto, BindingResult result, Model model) {

    examRepository.save(exam);
    model.addAttribute("examTitle", exam.getExamTitle());
    model.addAttribute("examGradeWorth", exam.getExamGradeWorth());
    model.addAttribute("subject", exam.getSubject()); 


    //I want to find the user selected value and set it to equal 
    //subjectname:
    String subjectName =;

//Here I will search the subjectRepository for the subjectName and set subject to equal the subject that was found.

    Subject subject = subjectRepository.findBySubjectName(subjectName);
 //then exam will be added to that subject as subject has a one to many relationship with exam.

    subject.addExam(exam);
subjectRepository.save(subject);

return "userProfile1";


}
 }

这是html。

<form action="#" th:action="@{/addExam}" th:object="${exam}" 
         method="post">
                     <div th:object="${subject}">
            <select th:field="*{subject}" class="form-control" id="subject"             
   name= "subject">
            <option value="">Select subject</option>

            <option 
                th:each="Subject : ${subjects}" 
                th:value="${Subject}" 
                th:text="${Subject}"></option>
         </div>
         <div>
                <table>
                    <tr>

                     <td><input type="text" th:field="*{examTitle}" /></td>

                    </tr>  
                    <tr>
                        <td> Exam grade worth </td>
                        <td><input th:field="*{examGradeWorth}" /></td>

                        </tr>