One more thing Max -
Can you show the code that created the arguments passed into your
method?
public void addChapterToBook(PersistenceManager pm, Key bookKey,
Chapter
chapter) {
Peace,
Scott
On Oct 27, 12:25 pm, "Max Ross (Google)" <[email protected]>
wrote:
> At what point are you checking to see if the parent property of the child is
> populated? I see that the newly created Chapter's Book property is
> populated when addChapterToBook() completes.
>
> Max
>
> On Tue, Oct 27, 2009 at 8:31 AM, stanlick <[email protected]> wrote:
>
> > This code saves the child for me, however, the parent property of the
> > child (in your case "book") is null in the child. I have tried
> > deliberately setting this child "parent" property myself to the parent
> > prior to commit, but nothing seems to work.
>
> > On Sep 28, 6:45 pm, Max Ross
> > <[email protected]<maxr%[email protected]>>
> > wrote:
> > > Hello again and welcome to Episode 3 of JDO/JPA Snippets That Work.
> > Today's
> > > episode is called......
>
> > > Updating A Bidrectional Owned One-To-Many With A New Child
>
> > > All the way back in episode one we demonstrated how to create both a
> > parent
> > > and a child of a bidirectional, owned, one-to-many relationship
> > > at the same time. This week we're going to see how to add a child to an
> > > existing parent. We'll use the same model objects we used in episode
> > one:
>
> > > 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 assume we've already created a book with a few chapters in the
> > > datastore and we want to add a brand new chapter to a Book with a given
> > id
> > > (we'll assume someone else is creating and closing an EntityManager named
> > > 'em' for us):
>
> > > public void addChapterToBook(EntityManager em, Key bookKey, Chapter
> > chapter)
> > > {
> > > em.getTransaction().begin();
> > > try {
> > > Book b = em.find(Book.class, bookKey);
> > > if (b == null) {
> > > throw new RuntimeException("Book " + bookKey + " not
> > found!");
> > > }
> > > b.getChapters().add(chapter);
> > > 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")
> > > @Order(extensions = @Extension(vendorName="datanucleus", key=
> > > "list-ordering", value="id asc"))
> > > 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 assume we've already created a book with a few chapters in the
> > > datastore and we want to add a brand new chapter to a Book with a given
> > id
> > > (we'll assume someone else is creating and closing a PersistenceManager
> > > named 'pm' for us):
>
> > > public void addChapterToBook(PersistenceManager pm, Key bookKey, Chapter
> > > chapter) {
> > > pm.currentTransaction().begin();
> > > try {
> > > // throws a runtime exception if book is not found
> > > Book b = pm.getObjectById(Book.class, bookKey);
> > > b.getChapters().add(chapter);
> > > pm.currentTransaction().commit();
> > > } finally {
> > > if (pm.currentTransaction().isActive()) {
> > > pm.currentTransaction().rollback();
> > > }
> > > }}
>
> > > --------------------------------
>
> > > The interesting thing about both of these examples is that we're not
> > making
> > > any explicit calls to save the new Chapter. We look up the Book
> > identified
> > > by the Key that was passed into the function and then we manipulate the
> > > persistent state of the object by manipulating the POJO that was returned
> > by
> > > em.fetch/pm.getObjectById. JPA and JDO both have mechanisms that allow
> > them
> > > to monitor the objects that you've looked up for changes. Ever wonder
> > what
> > > exactly the enhancer is doing to your classes? It's adding hooks so that
> > > the persistence framework gets notified when things change (among other
> > > things). This allows JPA and JDO to automatically flush your changes to
> > the
> > > datastore when you commit your transaction. If you wanted to modify the
> > > title of the Book or the number of pages in an existing Chapter the
> > approach
> > > would be exactly the same: Start a transaction, look up the Book, make
> > your
> > > changes, commit your transaction. Whether you're using JPA or JDO your
> > > changes will be persisted for you without any explicit calls to change
> > the
> > > persistent state. This is a prime example of how JPA and JDO facilitate
> > > "transparent persistence."
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