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." > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Google App Engine for Java" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/google-appengine-java?hl=en -~----------~----~----~----~------~----~------~--~---
