Java 使用play framework 2上的ManyToOne字段绑定表单请求

当我在模型“Item”中添加一个manytone字段时，我遇到了这个错误&我试图绑定相应的表单

Execution exception
[IllegalStateException: No value] at line 31

=> Item item = itemForm.bindFromRequest().get();

“项目”模型： 包装模型

@Entity 
public class Item extends Model {

    @Id
    public Long id;

    @ManyToOne
    @Constraints.Required
    @Formats.NonEmpty
    public Category category;

    @Constraints.Required
    public String title;

    @Constraints.Required
    public String content;

    public String picture;

    (..)    
}

表单视图：

    @helper.form(action = routes.Application.newItem(), 'id -> "item_form", 'method -> "POST", 'enctype -> "multipart/form-data"){
    <fieldset>
        @helper.inputText(
        itemForm("title"),
        '_label -> "Titre"  )

        @helper.select(
        itemForm("category"), 
        helper.options(Category.list),
        '_label -> "Categorie")

        @helper.textarea(
        itemForm("content"),
        '_label -> "Description")

         @helper.inputFile(
         field = itemForm("picture"), 
         '_display -> "Attachment", 
         '_label -> Messages("Image") )
         <input type="submit" value="Ajouter">

    </fieldset>
    }

---

考虑到类别模型具有id字段，类别字段的表单视图应为

@helper.select(
        itemForm("category.id"), 
        helper.options(Category.list),
        '_label -> "Categorie")

@helper.select(
        itemForm("category.id"), 
        helper.options(Category.list),
        '_label -> "Categorie")