Hibernate实体层次设计(三)

Table per subclass:

父类TItem单独映射到一张主表，为子类TBook、TDVD分别单独设立一张子表，子表中只包含子类所扩展的属性。同时，子表通过关系型数据库的外键相关联，如下图所示：

数据关系图如下：


1.表定义sql如下

use sample;

DROP TABLE T_DVD;
DROP TABLE T_Book;
DROP TABLE T_Item;

CREATE TABLE T_Item (
        id INT NOT NULL AUTO_INCREMENT
      , name VARCHAR(50)
      , manufacturer VARCHAR(50)
      , PRIMARY KEY (id)
);

CREATE TABLE T_Book (
        id INT NOT NULL
      , pagecount INT
      , PRIMARY KEY (id)
      , INDEX (id)
      , CONSTRAINT FK_T_Book_1 FOREIGN KEY (id)
                  REFERENCES T_Item (id)
);

CREATE TABLE T_DVD (
        id INT NOT NULL
      , regioncode VARCHAR(30)
      , PRIMARY KEY (id)
      , INDEX (id)
      , CONSTRAINT FK_T_DVD_1 FOREIGN KEY (id)
                  REFERENCES T_Item (id)
);

2.配置文件
TItem.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="cn.blogjava.start">

    <class name="TItem" table="T_Item" catalog="sample">
        <id name="id" column="id">
            <generator class="native" />
        </id>
        <property name="name" column="name"/>
        <property name="manufacturer" column="manufacturer"/>        
        
        <joined-subclass name="TDVD" table="T_DVD" catalog="sample">
            <key column="id" />
            <property name="regionCode" column="regioncode" />
        </joined-subclass>
        
        <joined-subclass name="TBook" table="T_Book" catalog="sample">
            <key column="id" />
            <property name="pageCount" column="pagecount" />
        </joined-subclass>
               
    </class>
</hibernate-mapping>

3.测试代码。

package cn.blogjava.start;

import java.util.Iterator;
import java.util.List;

import junit.framework.Assert;
import junit.framework.TestCase;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;


public class HibernateTest extends TestCase {
    
     Session session = null;

    protected void setUp() {
        try {
             Configuration config = new Configuration().configure();
             SessionFactory sessionFactory = config.buildSessionFactory();
             session = sessionFactory.openSession();
            
         } catch (HibernateException e) {
             e.printStackTrace();
         }        
     }

    protected void tearDown() {
        try {
             session.close();        
         } catch (HibernateException e) {
             e.printStackTrace();
         }        
     }    
      
    public void testInsert() {
         Transaction tran = null;
        try {
             tran = session.beginTransaction();
             TBook book1 = new TBook();
            
             book1.setManufacturer("电子工业");
             book1.setName("Java beginner");
             book1.setPageCount(501);
            
             TBook book2 = new TBook();
             book2.setManufacturer("机械工业");
             book2.setName("Thinking in java");
             book2.setPageCount(1201);
            
             TDVD dvd1 = new TDVD();
             dvd1.setManufacturer("columnibia");
             dvd1.setName("Lord king");
             dvd1.setRegionCode("5 area");
            
             TDVD dvd2 = new TDVD();
             dvd2.setManufacturer("sony");
             dvd2.setName("Forrest gump");
             dvd2.setRegionCode("3 area");            

             session.save(book1);
             session.save(book2);
             session.save(dvd1);
             session.save(dvd2);
             session.flush();
             tran.commit();
             Assert.assertEquals(book1.getId().intValue()>0, true);
             Assert.assertEquals(book2.getId().intValue()>0, true);
             Assert.assertEquals(dvd1.getId().intValue()>0, true);
             Assert.assertEquals(dvd2.getId().intValue()>0, true);
         } catch (HibernateException e) {
             e.printStackTrace();
             Assert.fail(e.getMessage());
            if(tran != null) {
                try {
                     tran.rollback();
                 } catch (Exception e1) {
                     e1.printStackTrace();
                 }
             }
         }
     }
    
    public void testSelect(){
         List list = session.createQuery(" from TItem").list();
        
         Iterator it = list.iterator();
        while (it.hasNext()) {
             TItem item = (TItem)it.next();
             System.out.println(item.getName());
            
         }
     }
}

4.在HQL Scratchpad中输入 From TItem，在Hibernate Dynamic Query Translator中观察到如下的SQL语句：
select
   titem0_.id as id16_,
   titem0_.name as name16_,
   titem0_.manufacturer as manufact3_16_,
   titem0_1_.regioncode as regioncode17_,
   titem0_2_.pagecount as pagecount18_,
  case
   when titem0_1_.id is not null then 1
   when titem0_2_.id is not null then 2
   when titem0_.id is not null then 0
  end as clazz_
from
   sample.T_Item titem0_
left outer join
   sample.T_DVD titem0_1_
   on titem0_.id=titem0_1_.id
left outer join
   sample.T_Book titem0_2_
   on titem0_.id=titem0_2_.id