Hibernate笔记_Mapping

1. 对Mapping的最好定义：

The mappings are applied to express the various different ways of forming associations in the underlying tables; there
is no absolutely correct way to represent them.

意思是不是：对象之间的关联 + 描述映射的annotation就可以推导出数据库结构。

2. mappedBy:

• The mappedBy attribute is mandatory on a bidirectional association and optional (being implicit)
  on a unidirectional association.

@Entity(name = "Email2")public class Email {    @Id    @GeneratedValue(strategy = GenerationType.IDENTITY)    Long id;    @Column    String subject;    @OneToOne(mappedBy = "email")    Message message;    public Email() {    }}@Entity(name = "Message2")public class Message {    @Id    @GeneratedValue(strategy = GenerationType.IDENTITY)    Long id;    @Column    String content;    @OneToOne    Email email;    public Message() {    }}

意思是：

1）Email 是主表，Message是从表。即Message表中有一外键是email_id。

2）(从表是关系的所有者)Only changes to the owner of an association will be honored in the database.

     意思是 message.setEmail(email);才会在数据库中创建主外键关系。完整的测试代码如下：

   

@Test    public void testImpliedRelationship() {        Long emailId;        Long messageId;        Session session = SessionUtil.getSession();        Transaction tx = session.beginTransaction();        Email email = new Email("Inverse Email");        Message message = new Message("Inverse Message");        // email.setMessage(message);        message.setEmail(email);        session.save(email);        session.save(message);        emailId = email.getId();        messageId = message.getId();        tx.commit();        session.close();        assertEquals(email.getSubject(), "Inverse Email");        assertEquals(message.getContent(), "Inverse Message");        assertNull(email.getMessage());        assertNotNull(message.getEmail());        session = SessionUtil.getSession();        tx = session.beginTransaction();        email = (Email) session.get(Email.class, emailId);        System.out.println(email);        message = (Message) session.get(Message.class, messageId);        System.out.println(message);        tx.commit();        session.close();        assertNotNull(email.getMessage());        assertNotNull(message.getEmail());    }

要点：

Table 4-1 shows how you can select the side of the relationship that should be made the owner of a bidirectional
association. Remember that to make an association the owner, you must mark the other end as being mapped by the other.
Table 4-1. Marking the Owner of an Association