Why the value of the foreign key is always zero?

I try to build some application using hibernate for persistence (my first one). The application has genres and books. The genre class has a set of books. But every book in the set has the idGenre (the foreign key) value 0. I think the mapping is not right. Please tell me where is the mistake. Thanks.

Here is the mapping:

<hibernate-mapping package = "model">

   <class name = "User" table="virtual_bookcase.users">
        <id name = "id" column = "id" type = "long">
            <generator class = "increment"/>
        </id>
        <property name = "username" column = "username" type = "string"/>
        <property name = "password" column = "password" type = "string"/>
   </class>

   <class name = "Genre" table = "virtual_bookcase.genres">
      <id name = "id" column = "idGenres" type = "long">
        <generator class = "increment"/>
      </id>   
      <property name = "name" column = "name" type = "string"/>
      <set name = "books" table = "virtual_bookcase.books" cascade = "all-delete-orphan">
        <key column = "idGenre" not-null = "true" />
        <one-to-many class = "Book"/>
      </set>
      <many-to-one name = "user" class = "User" column = "user_id" />
   </class>

   <class name = "Book" table = "virtual_bookcase.books">
        <id name = "id" column = "idBooks" type = "long">
         <generator class = "increment"/>
        </id>
        <property name = "title" column = "title" type = "string"/>
        <property name = "author" column = "author" type = "string"/>
        <property name = "publisher" column = "publisher" type = "string"/>
        <property name = "pages" column = "pages" type = "short"/>
        <property name = "borrowe开发者_C百科d" column = "borrowed" type = "byte"/>
        <property name = "borrowedTo" column = "borrowedTo" type = "string"/>
   </class>

</hibernate-mapping>

Here is where i load the books:

Session s = sessionFactory.openSession();

Query q = s.createQuery("FROM Book WHERE idGenre = ?").setLong(0, g.getId());

books = new TreeSet<Book>(q.list());

Here is the Book class:

public class Book implements Comparable<Book>
{
    private long id;
    private String title;
    private String author;
    private String publisher;
    private short pages;
    private byte borrowed;
    private String borrowedTo;
    private long idGenre;

public Book(){}
    public Book(String title, String author, String publisher, short pag, byte borrowed, String borrowedTo, long idGenre)
    {
        this.title = title;
        this.author = author;
        this.publisher = publisher;
        this.pages = pag;
        this.borrowed = borrowed;
        this.borrowedTo = borrowedTo;
        this.idGenre = idGenre;
    }

    public long getId()
    {
        return id;
    }

    public String getTitle()
    {
        return title;
    }

    public String getAuthor()
    {
        return author;
    }

    public String getPublisher()
    {
        return publisher;
    }

    public short getPages()
    {
        return pages;
    }

    public void setId(long id)
    {
        this.id = id;
    }

    public void setTitle(String title)
    {
        this.title = title;
    }

    public void setAuthor(String author)
    {
        this.author = author;
    }

    public void setPublisher(String publisher)
    {
        this.publisher = publisher;
    }

    public void setPages(short pages)
    {
        this.pages = pages;
    }

    public byte getBorrowed()
    {
        return borrowed;
    }

    public void setBorrowed(byte borrowed)
    {
        this.borrowed = borrowed;
    }

    public String getBorrowedTo()
    {
        return borrowedTo;
    }

    public void setBorrowedTo(String borrowedTo)
    {
        this.borrowedTo = borrowedTo;
    }

    public long getIdGenre()
    {
        return idGenre;
    }

    public void setIdGenre(long idGenre)
    {
        this.idGenre = idGenre;
    }

    @Override
    public String toString()
    {
        return title;
    }

    @Override
    public int compareTo(Book b)
    {
        if (this.title == b.getTitle() && this.author == b.getAuthor() && this.publisher == b.getPublisher() && this.pages == b.getPages())
            return 0;
        return 1;
    }

}

Here is how i make a new instance of a Book:

Book b = new Book("AddedBook", "A", "A", (short) 555, (byte) 0, "", 1);
Genre g = ((Genre) cmbGenres.getSelectedItem());
g.addBook(b);
control.saveGenre(g);

And the saveGenre(g) method is (without initialisation of SessionFactory and session and transaction):

   t = session.beginTransaction();
   Genre gr = (Genre) session.merge(g);
   t.commit();

Your mapping looks OK to me at first sight. My suspicion is that you may not properly associate your Book entities with the corresponding Genre. Could you post the code to create and persist Books?

Update: your code looks like it could work, although it is not clear why you merge your genre (if you work with your entities in detached state during conversations spanning multiple sessions, it's OK, otherwise may be overcomplicating the picture). And the definition of Genre.addBook is missing, but I assume you did it right :-)

My new observation is that you haven't mapped Book.idGenre. Try extending your mapping like this:

<class name = "Book" table = "virtual_bookcase.books">
    ...
    <many-to-one name="idGenre" column="idGenre" class="Genre" not-null="true"/>
</class>

As a side note, this also makes some of the attributes on the other side of the association redundant - you could simplify it like this:

  <set name = "books" cascade = "all-delete-orphan">
    <key column = "idGenre"/>
    <one-to-many class = "Book"/>
  </set>

Update2: oops, and one more thing: replace long idGenre with a Genre property in Book, like

private Genre genre;

and of course update getter/setter, and mapping accordingly.