Java 从thymeleaf对象获取值

TL:DR

与上面的问题非常相似，但我的值“thepublisher”将所选值传递为空，并且我已经完成了建议的解决方案。 我有一个图书表单（添加）页面，如下所示：

<form th:action="@{/book/save}"
          th:object="${book}" method="POST">
    
        <!-- Add hidden form field to handle update -->
        <input type="hidden" th:field="*{id}" />
        
        <input type="text" th:field="*{bookName}"
                class="form-control mb-4 col-4" placeholder="First name">

        <input type="text" th:field="*{bookDescription}"
                class="form-control mb-4 col-4" placeholder="Last name">

        <div class="form-group blu-margin">
            <select class="form-control"   th:field="${thepublisher}" id="dropOperator">
                <option value="0">select operator</option>
                <option th:each="publisher : ${thepublisher}" th:value="${publisher.id}" th:text="${publisher.publisherName}"></option>
            </select>
        </div>
            
        <button type="submit" class="btn btn-info col-2">Save</button>
                    
    </form>

 @Id
    @GeneratedValue(strategy= GenerationType.IDENTITY)
    @Column(name="id")
    private int id;

    @Column(name="book_name")
    private String bookName;

    @Column(name="book_sub_name")
    private String bookSubName;

    @Column(name="book_series_name")
    private String bookSeriesName;

    @Column(name="book_isbn")
    private String bookISBN;

    @Column(name="book_description")
    private String bookDescription;

    @OneToOne
    private Writer theWriter;


    @OneToOne
    private Publisher thePublisher;

我的出版者的代码如下（我不打算让writer简单化，我也会将其与book对象关联）：

我前面提到的添加页面的控制器代码

   @GetMapping("/showFormForAdd")
    public String showFormForAdd(Model theModel) {

        // create model attribute to bind form data
        BookDto theBook = new BookDto();
        List<Publisher> thePublisher = publisherService.findAll();
        theModel.addAttribute("book", theBook);
        theModel.addAttribute("thepublisher",thePublisher);

        return "book/book-form";
    }

我的主要问题是Publisher对象在传递到saveEmployee函数时始终为空。 我可以用出版商填满这个页面，这样就可以把我的价值观传递到那里

   @GetMapping("/showFormForAdd")
    public String showFormForAdd(Model theModel) {

        // create model attribute to bind form data
        BookDto theBook = new BookDto();
        List<Publisher> thePublisher = publisherService.findAll();
        theModel.addAttribute("book", theBook);
        theModel.addAttribute("thepublisher",thePublisher);

        return "book/book-form";
    }

@PostMapping("/save")
    public String saveEmployee(@ModelAttribute("book") BookDto theBook) {

        // save the employee

        Book NewItem = new Book();

        NewItem.setBookDescription(theBook.getBookDescription());
        NewItem.setBookName(theBook.getBookName());
        // use a redirect to prevent duplicate submissions
        return "redirect:/book/list";
    }