Hibernate集合映射之IDBag

假设Team和Student是1对多的关系，而student中只有team_id和name两个属性，我们可以不建立Student实体类，采用element的方式，由于bag可以允许重复，所以，在我们根据team删除其下某一个student的时候，hibernate不知道具体要删除哪一条数据，所以，只有讲team下所有student全部删除，然后再重新插入不应该删除的数据，这样会对效率有很大影响，针对这种情况，可以采用idbag标签，在student中加一个字段cid来唯一标识每一个student

数据库结构：

 

create table teamIDBag (id varchar(32),teamname varchar(32));
create table studentIDBag(team_id varchar(32),name varchar(32),cid varchar(32));

Pojo:

 

package Collection.IDBag;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class Team ...{
   private String id;
   private String teamname;
   private List students=new ArrayList();
public String getId() ...{
    return id;
}
public void setId(String id) ...{
    this.id = id;
}
public String getTeamname() ...{
    return teamname;
}
public void setTeamname(String teamname) ...{
    this.teamname = teamname;
}
public List getStudents() ...{
    return students;
}
public void setStudents(List students) ...{
    this.students = students;
}

}

 Team.hbm.xml

 

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- 
    Mapping file autogenerated by MyEclipse - Hibernate Tools
-->
<hibernate-mapping>
<class name="Collection.IDBag.Team" table="teamIDBag" >
    <id name="id" unsaved-value="null">
      <generator class="uuid.hex"></generator>
    </id>
    <property name="teamname" type="string" column="teamname"></property>
   
    <idbag name="students" table="studentIDBag" cascade="all,delete-orphan" >
      <collection-id type="string" column="cid">
        <generator class="uuid.hex"></generator>
      </collection-id>
      <key column="team_id"></key>
      <element type="string" column="name"></element>
    </idbag>
</class>

</hibernate-mapping>

 

Hibernate.cfg.xml

 

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
          "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
          "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<!-- Generated by MyEclipse Hibernate Tools.                   -->
<hibernate-configuration>

<session-factory>
    <property name="connection.username">root</property>
    <property name="connection.url">
        jdbc:mysql://localhost:3306/schoolproject?characterEncoding=gb2312&amp;useUnicode=true
    </property>
    <property name="dialect">
        org.hibernate.dialect.MySQLDialect
    </property>
    <property name="myeclipse.connection.profile">mysql</property>
    <property name="connection.password">1234</property>
    <property name="connection.driver_class">
        com.mysql.jdbc.Driver
    </property>
    <property name="hibernate.dialect">
        org.hibernate.dialect.MySQLDialect
    </property>
    <property name="hibernate.show_sql">true</property>
    <property name="current_session_context_class">thread</property>
    <property name="jdbc.batch_size">15</property>
    <mapping resource="Collection/IDBag/Team.hbm.xml" />




</session-factory>

</hibernate-configuration>

 

测试代码：

 

package Collection.IDBag;


import java.io.File;
import java.util.Map;

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

public class Test ...{


    public static void main(String[] args) ...{

        String filePath=System.getProperty("user.dir")+File.separator+"src/Collection/IDBag"+File.separator+"hibernate.cfg.xml";
        File file=new File(filePath);
        System.out.println(filePath);
        SessionFactory sessionFactory=new Configuration().configure(file).buildSessionFactory();
        Session session=sessionFactory.openSession();
        Transaction t=session.beginTransaction();
        
    
        
//        Team team1=new Team();
//        team1.setTeamname("team1");
//        
//        Team team2=new Team();
//        team2.setTeamname("team2");
//        
//       
//       
//
//        //由于Bag映射可以重复，所以这里重复保存student
//        team1.getStudents().add("tom1");
//        team1.getStudents().add("tom1");
//        team2.getStudents().add("tom2");
//        
//        
//        
//        session.save(team1);
//        session.save(team2);
        

        Team t1=(Team)session.createQuery("from Team t where t.teamname='team1'").uniqueResult();
        t1.getStudents().remove(0);
        t.commit();
        
    }

}

 

运行结果：

Hibernate: select team0_.id as id0_, team0_.teamname as teamname0_ from teamIDBag team0_ where team0_.teamname='team1'
Hibernate: select students0_.team_id as team1_0_, students0_.name as name0_, students0_.cid as cid0_ from studentIDBag students0_ where students0_.team_id=?
Hibernate: delete from studentIDBag where cid=?

可以红色部分，删除student时是根据cid删的，如果不使用idbag,则会出类似delete from studentIDBag where team_id=?的操作，然后会接着跟一个insert操作，来恢复本不应该删除的数据