Just to show how easy this sort of query and mapping is in Twig.
Notice the lack of Keys - the Book id is optional.
public class Book {
private long id;
private String title;
@Entity(Child)
private List<Chapter> chapters = new ArrayList<Chapter>();
}
The @Entity(Child) annotation applies to all the elements of the
collection
public class Chapter {
@Key
private String title;
private int numPages;
@Entity(Parent)
private Book book;
}
The Chapter title has been configured to be the key name so it is
automatically encoded in the key.
Getting the book is as easy as:
session.find(Chapter.class, "War and Peace").getBook()
Notice that because Twig was written from scratch using generics the
result of find() is known to be a Chapter so we can then just call
getBook() on the result.
http://code.google.com/p/twig-persist/
On 3 Nov 2009, at 00:03, Solvek wrote:
>
> 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 <[email protected]> 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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