Part1 - JDO/JPA建立一个双向，有主，一对多(one-to-many)的关系

你好，欢迎来到第一部分的"JDO/JPA Snippets That Work"!

建立一个双向的有主的一对多关系

假设你正在建立一个图书目录应用程序，你想为书和章节建模：书本包含章节，章节不能独立于书本存在，所以如果你删除一本书，他的章节也会跟着自动删除。同时你想每一个章节的实例都一个指向拥有这个章节的书本的引用。听起来，这正是双向的，有主的，一对对多的关系。

首先我们将建立我们的模型，之后我将示范建立一本带有2个章节的书。

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 
} 

现在让我们建立一个带有两个章节的书本（我们假设有人已经为我们建立和关闭一个叫'em'的EntityManager）

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 
} 

现在让我们建立一个带有两个章节的书本（我们假设有人已经为我们建立和关闭一个叫'em'的EntityManager）

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(); 
    } 
}

英语原文：
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(); 
    }
} 

转载自:http://groups.google.com/group/google-appengine-java/browse_thread/thread/54c83dc6242fd633

说明:上面的中文翻译是我用自己能理解的方式翻译出来的，并不是逐字翻译，只作为自己参考用，并不十分精确。