六、hibernate表与表之间的关系（多对多关系）

多对多关系

创建实体类和对应映射文件

Student.java

 package com.qf.entity;

 import java.util.HashSet;
 import java.util.Set;

 public class Student {

     private Long sid;
     private String sname;
     //一个学生可以有很多个老师
     private Set<Teacher> teachs = new HashSet<>();

     public Long getSid() {
         return sid;
     }
     public void setSid(Long sid) {
         this.sid = sid;
     }

     public String getSname() {
         return sname;
     }
     public void setSname(String sname) {
         this.sname = sname;
     }

     public Set<Teacher> getTeachs() {
         return teachs;
     }
     public void setTeachs(Set<Teacher> teachs) {
         this.teachs = teachs;
     }
     @Override
     public String toString() {
         return "Student [sid=" + sid + ", sname=" + sname + ", teachs=" + teachs + "]";
     }

 }

Student.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
	<!-- 配置表与实体的映射关系 -->
	<class name="com.qf.entity.Student" table="student">
		<id name="sid" column="sid">
			<generator class="native"></generator>
		</id>
		<property name="sname" column="sname"/>
		<set name="teachs" table="teach_stu" >
			<key column="sid"/>
			<many-to-many class="com.qf.entity.Teacher" column="tid" />
		</set>
	</class>
</hibernate-mapping>

Teacher.java

 package com.qf.entity;

 import java.util.HashSet;
 import java.util.Set;

 public class Teacher {
     private long tid;
     private String tname;
     private String course;
     //一个教师可以有多个学生
     private Set<Student> stus = new HashSet<Student>();

     public Set<Student> getStus() {
         return stus;
     }
     public void setStus(Set<Student> stus) {
         this.stus = stus;
     }
     public long getTid() {
         return tid;
     }
     public void setTid(long tid) {
         this.tid = tid;
     }
     public String getTname() {
         return tname;
     }
     public void setTname(String tname) {
         this.tname = tname;
     }
     public String getCourse() {
         return course;
     }
     public void setCourse(String course) {
         this.course = course;
     }
     @Override
     public String toString() {
         return "Teacher [tid=" + tid + ", tname=" + tname + ", course=" + course + "]";
     }

 }

Teacher.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
	<!-- 配置表与实体的映射关系 -->
	<class name="com.qf.entity.Teacher" table="teacher">
		<id name="tid" column="tid">
			<generator class="native"/>
		</id>
		<property name="tname" column="tname"/>
		<property name="course" column="course"/>
		<set name="stus" table="teach_stu">
			<key column="tid"></key>
			<many-to-many class="com.qf.entity.Student" column="sid"/>
		</set>
	</class>
</hibernate-mapping>　

测试

@Test
public void test() {
	Session session = SessionFactoryUtil.getSession();
	Transaction tx = session.beginTransaction();

	//创建两个教师对象，三个学生对象
	//学生选择老师
	Teacher t1 = new Teacher();
	t1.setTname("张老师");
	Teacher t2 = new Teacher();
	t2.setTname("李老师");
	Student s1 = new Student();
	s1.setSname("张三");
	Student s2 = new Student();
	s2.setSname("李四");
	Student s3 = new Student();
	s3.setSname("王五");

	//学生和老师之间建立双向关系
	//多对多关系建立双向关系后，双方必须有一方放弃维护外键关系权（inverse），否则会因为中间表主键重复抛出异常
	s1.getTeachs().add(t1);
	s1.getTeachs().add(t2);
	s3.getTeachs().add(t1);
	s2.getTeachs().add(t2);
	t1.getStus().add(s1);
	t1.getStus().add(s3);
	t2.getStus().add(s2);
	t2.getStus().add(s1);

	session.save(s1);
	session.save(s2);
	session.save(s3);
	session.save(t1);
	session.save(t2);

	tx.commit();
}

-------------------------------------console-------------------------------------

Hibernate: alter table teach_stu drop foreign key FKsrw3nh8oe5xhmqqxm2qng952t
Hibernate: alter table teach_stu drop foreign key FKtc48sy6cweqkufd4c777i8vsp
Hibernate: drop table if exists student
Hibernate: drop table if exists teach_stu
Hibernate: drop table if exists teacher
Hibernate: create table student (sid bigint not null auto_increment, sname varchar(255), primary key (sid))
Hibernate: create table teach_stu (tid bigint not null, sid bigint not null, primary key (sid, tid))
Hibernate: create table teacher (tid bigint not null auto_increment, tname varchar(255), course varchar(255), primary key (tid))
Hibernate: alter table teach_stu add constraint FKsrw3nh8oe5xhmqqxm2qng952t foreign key (sid) references student (sid)
Hibernate: alter table teach_stu add constraint FKtc48sy6cweqkufd4c777i8vsp foreign key (tid) references teacher (tid)
11:21:37,135  INFO SchemaExport:464 - HHH000230: Schema export complete
Hibernate: insert into student (sname) values (?)
Hibernate: insert into student (sname) values (?)
Hibernate: insert into student (sname) values (?)
Hibernate: insert into teacher (tname, course) values (?, ?)
Hibernate: insert into teacher (tname, course) values (?, ?)
Hibernate: insert into teach_stu (sid, tid) values (?, ?)
Hibernate: insert into teach_stu (sid, tid) values (?, ?)
Hibernate: insert into teach_stu (sid, tid) values (?, ?)
Hibernate: insert into teach_stu (sid, tid) values (?, ?)
Hibernate: insert into teach_stu (tid, sid) values (?, ?)
11:21:37,270  WARN SqlExceptionHelper:127 - SQL Error: 1062, SQLState: 23000
11:21:37,271 ERROR SqlExceptionHelper:129 - Duplicate entry '1-1' for key 'PRIMARY'
11:21:37,271  INFO AbstractBatchImpl:193 - HHH000010: On release of batch it still contained JDBC statements
11:21:37,272 ERROR SessionImpl:2994 - HHH000346: Error during managed flush [could not execute statement]

原因

　　测试中因为建立了双向关系，所以teacher和student都会去维护外键，导致相同的（1，1）会两次插入中间关系表teach_stu中

解决办法

　　一方放弃维护外键关系权。一般被动主体放弃维护外键权，比如学生选择老师，老师放弃维护外键权；老师选择学生，学生放弃维护外键权

修改Teacher.hbm.xml：<set name="stus" table="teach_stu" inverse="true">

-------------------------------------console-------------------------------------

Hibernate: alter table teach_stu drop foreign key FKsrw3nh8oe5xhmqqxm2qng952t
Hibernate: alter table teach_stu drop foreign key FKtc48sy6cweqkufd4c777i8vsp
Hibernate: drop table if exists student
Hibernate: drop table if exists teach_stu
Hibernate: drop table if exists teacher
Hibernate: create table student (sid bigint not null auto_increment, sname varchar(255), primary key (sid))
Hibernate: create table teach_stu (tid bigint not null, sid bigint not null, primary key (sid, tid))
Hibernate: create table teacher (tid bigint not null auto_increment, tname varchar(255), course varchar(255), primary key (tid))
Hibernate: alter table teach_stu add constraint FKsrw3nh8oe5xhmqqxm2qng952t foreign key (sid) references student (sid)
Hibernate: alter table teach_stu add constraint FKtc48sy6cweqkufd4c777i8vsp foreign key (tid) references teacher (tid)
11:30:43,077  INFO SchemaExport:464 - HHH000230: Schema export complete
Hibernate: insert into student (sname) values (?)
Hibernate: insert into student (sname) values (?)
Hibernate: insert into student (sname) values (?)
Hibernate: insert into teacher (tname, course) values (?, ?)
Hibernate: insert into teacher (tname, course) values (?, ?)
Hibernate: insert into teach_stu (sid, tid) values (?, ?)
Hibernate: insert into teach_stu (sid, tid) values (?, ?)
Hibernate: insert into teach_stu (sid, tid) values (?, ?)
Hibernate: insert into teach_stu (sid, tid) values (?, ?)

数据成功插入数据库

级联保存

保存学生级联保存教师

主体是学生，所以修改学生的映射文件Student.hbm.xml

<hibernate-mapping>
	<!-- 配置表与实体的映射关系 -->
	<class name="com.qf.entity.Student" table="student">
		<id name="sid" column="sid">
			<generator class="native"></generator>
		</id>
		<property name="sname" column="sname"/>
		<set name="teachs" table="teach_stu" cascade="save-update" >
			<key column="sid"/>
			<many-to-many class="com.qf.entity.Teacher" column="tid" />
		</set>
	</class>
</hibernate-mapping>

测试方法

@Test
public void test() {
	Session session = SessionFactoryUtil.getSession();
	Transaction tx = session.beginTransaction();

	//保存学生级联保存教师
	Teacher t1 = new Teacher();
	t1.setTname("张老师");
	Student s1 = new Student();
	s1.setSname("张三");

	t1.getStus().add(s1);
	s1.getTeachs().add(t1);

	session.save(s1);

	tx.commit();
}

-------------------------------------console-------------------------------------

Hibernate: create table student (sid bigint not null auto_increment, sname varchar(255), primary key (sid))
Hibernate: create table teach_stu (tid bigint not null, sid bigint not null, primary key (sid, tid))
Hibernate: create table teacher (tid bigint not null auto_increment, tname varchar(255), course varchar(255), primary key (tid))
Hibernate: alter table teach_stu add constraint FKsrw3nh8oe5xhmqqxm2qng952t foreign key (sid) references student (sid)
Hibernate: alter table teach_stu add constraint FKtc48sy6cweqkufd4c777i8vsp foreign key (tid) references teacher (tid)
14:22:37,418  INFO SchemaExport:464 - HHH000230: Schema export complete
Hibernate: insert into student (sname) values (?)
Hibernate: insert into teacher (tname, course) values (?, ?)
Hibernate: insert into teach_stu (sid, tid) values (?, ?

级联删除（一般不使用）

想以哪一方为主体删除另一方，就在哪一方映射文件中配置cascade属性：

　　<set name="teachs" table="teach_stu" cascade="delete" >