Hibernate - 多对多级联删除的问题
Hibernate多对多的例子不少，但仔细一看，大多数都是保存的，删除谈的少，但问题还不少，因此有必须简单测试一下，以下我们来个简单的多对多关系建立老师Teacher 与课程Course 是一个多对多的关系，Pojo与XMl配置如下。
以下为 Pojo代码
=================================Course Pojo================================
package com.domain;
import java.util.HashSet;
import java.util.Set;
/**
* @author dengshaohua
* @create_date 2011-09-14
*/
publicclass Course {
private String id;
private String name;
private Set<Teacher> teachers = new HashSet<Teacher>();
public String getId() {
returnid;
}
publicvoid setId(String id) {
this.id = id;
}
public String getName() {
returnname;
}
publicvoid setName(String name) {
this.name = name;
}
public Set<Teacher> getTeachers() {
returnteachers;
}
publicvoid setTeachers(Set<Teacher> teachers) {
this.teachers = teachers;
}
}
================================Teacher Pojo===============================
package com.domain;
import java.util.HashSet;
import java.util.Set;
/**
* @author dengshaohua
* @create_date 2011-09-14
*/
publicclass Teacher {
private String id;
private String name;
private Set<Course> courses = new HashSet<Course>();
public String getId() {
returnid;
}
publicvoid setId(String id) {
this.id = id;
}
public String getName() {
returnname;
}
publicvoid setName(String name) {
this.name = name;
}
public Set<Course> getCourses() {
returncourses;
}
publicvoid setCourses(Set<Course> courses) {
this.courses = courses;
}
}
XML配置文件如下
=================================Course 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>
<class name="com.domain.Course" table="tbl_course"
batch-size="100" dynamic-insert="true"
dynamic-update="true">
<id name="id" column="id">
<generator class="uuid" />
</id>
<property name="name" column="name" type="string" />
<set access="property" lazy="true" inverse="false"
cascade="save-update" name="teachers"
batch-size="10" fetch="select"
table="tbl_teacher_course">
<key column="fk_course_id" />
<many-to-many class="com.domain.Teacher"
column="fk_teacher_id" />
</set>
</class>
</hibernate-mapping>
=================================Teacher 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>
<class name="com.domain.Teacher" table="tbl_teacher"
batch-size="100" dynamic-insert="true"
dynamic-update="true">
<id name="id" column="id">
<generator class="uuid" />
</id>
<property name="name" column="name" type="string" />
<set access="property" lazy="true" inverse="true"
cascade="save-update" name="courses"
batch-size="10" fetch="select"
table="tbl_teacher_course">
<key column="fk_teacher_id" />
<many-to-many class="com.domain.Course"
column="fk_course_id" />
</set>
</class>
</hibernate-mapping>
先往数据库里插入一些记录
publicvoid testSave() {
Session session = HibernateSessionFactory.getSession();
session.beginTransaction();
// create course
Course c1 = new Course();
Course c2 = new Course();
c1.setName("C");
c2.setName("Java");
// create teacher
Teacher t1 = new Teacher();
Teacher t2 = new Teacher();
t1.setName("Leo");
t2.setName("Rose");
// create relationship
c1.getTeachers().add(t1);
c1.getTeachers().add(t2);
t1.getCourses().add(c1);
t2.getCourses().add(c1);
// 因为主控方级联设置为save-update,如果设置为none,则下面被注释的代码需要开启，否则会报错
//session.save(t1);
//session.save(t2);
session.save(c1);
session.getTransaction().commit();
session.close();
}
下面是测试的一些结果：
1. 如果cascade不管主控方设置还是被控方设置成 all, delete等与delete级联删除有关即可，两端以及中间表的记录都会被删除，通常这样的需要是很少的，因此，如果你要这样的情况，只要简单设置成all, delete就可以轻松的将关系以及两端的记录删除的干干净净。
2. 只想删除某一端的记录以及中间的表的关联信息。这种需求通常是很常见的。这个时候cascade的设置是除与delete有关的任何级联约束。
以下是删除心得：
如果删除的是主控方，只需要简单的删除这条记录，级联关系以及主控方的记录同时删除，但被控方的记录仍然存在。因此只对主控方的多对多删除是最简单，直接的。代码如下：
publicvoid testDelete() {
String id = "402881ee175f04be01175f04c05d0001";
Session session = HibernateSessionFactory.getSession();
session.beginTransaction();
Course c1 = (Course) session.get(Course.class, id);
session.delete(c1);
session.getTransaction().commit();
session.close();
}
如果你这个时候想直接删除被控方，那么很遗憾的告诉你，你只做到了一半，你只是简单的把被控方的记录删除了，关联关系仍然存在中间表里，系统随时会因为你的关联访问报错，代码如下：
publicvoid testDeleteByInverse() {
String id = "402881ee175a2e7c01175a2e7ead0003";
Session session = HibernateSessionFactory.getSession();
session.beginTransaction();
Teacher t1 = (Teacher) session.get(Teacher.class, id);
session.delete(t1);
session.getTransaction().commit();
session.close();
}
如果想既想删除被控方，双想删除关联，请看下面代码:
publicvoid testDeleteByInverse2() {
String id = "402881ee175f04be01175f04c06c0002";
Session session = HibernateSessionFactory.getSession();
session.beginTransaction();
Teacher t1 = (Teacher) session.get(Teacher.class, id);
Set<Course> cs = t1.getCourses();
for (Course c : cs) {
c.getTeachers().remove(t1);
}
session.delete(t1);
session.getTransaction().commit();
session.close();
}