Now. How to retrieve a book and its chapters having only Chapter's
title (imagine all titles in whole systems are unique)
On Sep 15, 1:07 am, Max Ross <maxr+appeng...@google.com> wrote:
> Hello hello and welcome to the very first installment of JDO/JPA Snippets
> That Work!
>
> Creating A Bidrectional Owned One-To-Many
>
> 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:
> @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:
>
> @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();
> }
>
> }
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