How to correctly assign a value to an object in thymleaf?

Question

Java

I have an HTML form that has options (select box) for the categoryName field of a Product Object. The value for options is taken from the Category object and should be assigned to the categoryName field of the Product object,
but it doesn't, what could be the problem?

html:

<form action="#" th:action="@{/save}" th:object="${product}" method="post">
        <table border="0" cellpadding="10">
            <tr>
                <td>Наименования:</td>
                <td><input required type="text" th:field="*{name}"/></td>
            </tr>
            <tr>
                <td>Описание:</td>
                <td><input required type="text" th:field="*{description}"/>

                </td>
            </tr>
            <tr>
                <td>Категория:</td>
                <td>
                    <input required type="text" th:field="*{category}"/>

                    <!-- Проблема! значение из option не происвается в product-->
                    <select class="form-select form-select-sm" aria-label=".form-select-sm example">
                        <option selected>Выберите котегорию</option>
                        <option th:each="category : ${categoryList}" value="${category.id}"
                                th:text="${product.category}"></option>
                    </select>

                </td>
            </tr>
            <tr>
                <td>Цена:</td>
                <td><input required type="number" th:field="*{price}" pattern='[0-9]+(\\.[0-9][0-9]?)?'/></td>
            </tr>
            <tr>
                <td colspan="2">
                    <button type="submit" class="btn btn-primary">Сохранить</button>
                </td>
            </tr>
        </table>
    </form>

Controller:

// добавить продукт
  @RequestMapping("/new_product")
  public String showNewProductForm(Model model) {
    Product product = new Product();
    List<Category> category = categoryService.getAll();
    model.addAttribute("categoryList", category);
    model.addAttribute("product", product);

    return "new_product";
  }

Answer 1

Good afternoon.
First, make sure the specified string returns something. Add a breakpoint and see if there is a list of categories

List<Category> category = categoryService.getAll();

I'm not exactly sure, but can the names of the two fields somehow conflict ?! We should check this moment. Try changing it to something else
th:field="*{category}"
in the block below . category :For example, catand accordingly, change it in the corresponding lines too.

<option th:each="category : ${categoryList}" value="${category.id}"
                                th:text="${product.category}"></option>

You mean here, it is not assigned here? th:text="${product.category}"
If yes, then it will not be assigned, because in this case you only get the value product.category, and since product is new: Product product = new Product();, then there will also be no assigned category for it either.