hibernate 数据关联多对多 4.1

多对多,必须有一张关系表来维持关系

数据库student,teacher student_teacher 三张表

但是在pojo中只需要建立student和teacher两个类,除非关系表也代表某种业
务(比如订单中,除了产品还会记录价格,数量等),但是这时候性质也就变了,成了两个一对多
...好好想想...

//分别Set
public class Student {
    private Integer id;
    private String name;
    private Set<Teacher> teachers;
}
 
public class Teacher {
    private Integer id;
    private String name;
    private Set<Student> students;
}

xxx.hbm.xml配置

<?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 package="com.kaishengit.pojo">
 
    <class name="Teacher" table="teacher">
        <id name="id">
            <generator class="native"/>
        </id>
 
        <property name="name"/>
        <!-- name="students"属性名,table="student_teacher"关系表名,这里的
                        order-by表示查询的顺序,指查询关系表中id的倒序-->
        <set name="students" table="student_teacher" inverse="true" order-by="id desc" >
            <!-- tid表示teacher在关系表中的外键名 -->
            <key column="tid"/>
            <!-- class="Student"指定Set集合中的类是Student,column="sid"关系表中的外键
                order-by表示查询的顺序,指查询Student中id的倒序-->
            <many-to-many class="Student" column="sid" order-by="id desc,name asc"/>
        </set>
 
    </class>
 
</hibernate-mapping>

<?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 package="com.kaishengit.pojo">
 
    <class name="Student" table="student">
        <id name="id">
            <generator class="native"/>
        </id>
 
        <property name="name"/>
        <!-- name="teachers"指属性, -->
        <set name="teachers" table="student_teacher">
            <!-- student在这个关系表中的外键 -->
            <key column="sid"/>
            <many-to-many class="Teacher" column="tid"/>
        </set>
 
    </class>
 
</hibernate-mapping>

在hibernate.cfg.xml中添加这两项

程序执行

Teacher t1 = new Teacher();
        t1.setTeachername("t1");
        Teacher t2 = new Teacher();
        t2.setTeachername("t2");
 
        Student s1 = new Student();
        s1.setStudentname("s1");
        Student s2 = new Student();
        s2.setStudentname("s2");
 
        Set<Teacher> t= new HashSet<Teacher>();
        t.add(t1);
        t.add(t2);
        s1.setTeachers(t);
        s2.setTeachers(t);
 
        Set<Student> stu = new HashSet<Student>();
        stu.add(s1);
        stu.add(s2);
        t1.setStudents(stu);
        t2.setStudents(stu);
 
        session.save(s1);
        session.save(s2);
        session.save(t1);
        session.save(t2);

两方都维护关系的时候,关系表中就会插入额外一倍的无用数据
这是错误的,所以多对多必须指明一方放弃维护 inverse="true"

<set name="students" table="student_teacher" inverse="true" >
            <key column="tid"/>
            <many-to-many class="Student" column="sid" order-by="id desc,name asc"/>
        </set>