Vaadin SQLContainer参考-如何实现外键关系？

Question

我不知道如何使用Vaadin的SQLContainer实现外键关系。 假设我有两个表：

1. 带有属性title和* author_id *的书
2. 具有属性* id_author，first_name *和* last_name *的作者

当然，BOOK.author_id引用了AUTHOR.id，我已经创建了一个具有所有必要约束的MySQL-DB。 此外，我创建了容器并按如下方式填充它们：

private static void initContainers() {
    try {
        /* TableQuery and SQLContainer for book -table */
        TableQuery q1 = new TableQuery("book", connectionPool);
        bookContainer = new SQLContainer(q1);

        /* TableQuery and SQLContainer for author -table */
        TableQuery q2 = new TableQuery("author", connectionPool);
        authorContainer = new SQLContainer(q2);


    } catch (SQLException e) {
        e.printStackTrace();
    }
}

@SuppressWarnings("unchecked")
public boolean addBook(Book book) {
    Object id_author = authorContainer.addItem();
    authorContainer.getContainerProperty(id_author,  "first_name").setValue(book.getAuthor().getFirstName());
    authorContainer.getContainerProperty(id_author, "last_name").setValue(book.getAuthor().getLastName());      

    Object id = bookContainer.addItem();   
    bookContainer.getContainerProperty(id, "title").setValue(book.getTitle());         

//HERE IS MY TRY FOR ADDING THE REFERENCE (FOREIGN KEY RELATION)
    bookContainer.addReference(authorContainer, "author_id", "id_author");         
    authorContainer.setReferencedItem("id_author", "author_id", bookContainer);
    bookContainer.getContainerProperty(id, "author").setValue(bookContainer.getReferencedItem("id_author", authorContainer)); 

    try {
        authorContainer.commit();
        bookContainer.commit();
        return true;
    } catch (UnsupportedOperationException e) {
        e.printStackTrace();
    } catch (SQLException e) {
        e.printStackTrace();
    }
    return false;
}

谁能告诉我我在这里做错了什么？ 在添加引用之前，我也尝试提交author-container，但这也不起作用。

我总是在* bookContainer.getReferencedItem（“ id_author”，authorContainer）*上获得NullPointerException

Answer 1

尝试authorContainer.getReferencedItem("id_author", bookContainer)