Creating A Bidrectional Owned One-To-Many

(This was originally posted to the Google App Java Google Group on September 14, 2009)

Suppose you're building a book catalog application and you want to model books and chapters.  Books contain chapters.  A chapter cannot exist without a book, so if you delete a book you want its chapters automatically deleted along with it.  You also want to each chapter to have a reference to the book that owns it.  Sounds like a bidrectional, owned, one-to-many relationship is just the thing.  First we'll set up our model objects and then we'll add some code to create a Book with 2 Chapters.

JPA:
import com.google.appengine.api.datastore.Key;
//...

@Entity
public class Book {
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Key id;

    private String title;

    @OneToMany(mappedBy = "book", cascade = CascadeType.ALL)
    private List<Chapter> chapters = new ArrayList<Chapter>();

    // getters and setters
}


@Entity
public class Chapter {
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Key id;

    private String title;
    private int numPages;

    @ManyToOne(fetch = FetchType.LAZY)
    private Book book;

    // getters and setters
}

Now let's create a book with two chapters (we'll assume someone else is creating and closing an EntityManager named 'em' for us):

Book b = new Book();
b.setTitle("JPA 4eva");
Chapter c1 = new Chapter();
c1.setTitle("Intro");
c1.setNumPages(10);
b.getChapters().add(c1);
Chapter c2 = new Chapter();
c2.setTitle("Configuration");
c2.setNumPages(9);
b.getChapters().add(c2);

em.getTransaction().begin();
try {
    em.persist(b);
    em.getTransaction().commit();
} finally {
    if (em.getTransaction().isActive()) {
        em.getTransaction().rollback();
    }
}


JDO:

import com.google.appengine.api.datastore.Key;

//...

@PersistenceCapable(identityType = IdentityType.APPLICATION, detachable = "true")
public class Book {
 
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key id;

    private String title;

    @Persistent(mappedBy = "book")
    @Element(dependent = "true")
    private List<Chapter> chapters = new ArrayList<Chapter>();

    // getters and setters
}

@PersistenceCapable(identityType = IdentityType.APPLICATION, detachable = "true")
public class Chapter {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key id;

    private String title;
    private int numPages;

    @Persistent
    private Book book;

    // getters and setters
}

Now let's create a book with two chapters (we'll assume someone else is creating and closing a PersistenceManager named 'pm' for us):

Book b = new Book();
b.setTitle("JDO 4eva");
Chapter c1 = new Chapter();
c1.setTitle("Intro");
c1.setNumPages(10);
b.getChapters().add(c1);
Chapter c2 = new Chapter();
c2.setTitle("Configuration");
c2.setNumPages(9);
b.getChapters().add(c2);

pm.currentTransaction().begin();
try {
    pm.makePersistent(b);
    pm.currentTransaction().commit();
} finally {
    if (pm.currentTransaction().isActive()) {
        pm.currentTransaction().rollback();
    }
}