How to I get the user selected value from a drop-d

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

I have a form where the user is creating a new exam and in the form the user selects a subject from a drop down menu. This drop down contains strings of subjects not actual subject objects. In my program there are actual subject objects which have a one to many relationship with exam.

How do I find what value the user selected? I want to add it to the model so that I can create an exam object and add it to the database.

Once the subject string that the user selected is added to the model how do I find the value the user selected? As I want to set what they selected to equal subject Name and then search the subject repository for a subject with the same name so that the exam can be added to that subject.

Hopefully my code will make this a bit more clear.

I am getting this error. Neither Binding Result nor plain target object for bean name subject available as request attribute.

This the code form my Controller

@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";


}
 }

This is the 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>

I don't think you need to use two th:object. Just use th:value.

For instance, in the next example you are sending two objects and setting name in student object and status in exam object. (It isn't a representation of your model. It's just an example).

<form th:action="@{/addExam}" method="post">
      <input type="text" th:value="${student.name}" name="name"/>
      <input type="text" th:value="${exam.status}" name="status"/>
      <button type="submit">Go</button>
</form>

th:object would have been useful if there are a bunch of exam fields.

Keep in mind that the name of the fields have to match the bean field name.

So, your HTML will looks like this more or less:

<form action="#" th:action="@{/addExam}" method="post">
      <div>
          <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="*{exam.examTitle}" /></td>

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

                        </tr>

Let me know if it works!.