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>

巴特西

hibernate 数据关联多对多 4.1

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