Hibernate读书笔记Hibernate的关联映射之NN关联映射

          四、N—N关联关系

         1.1单向N-N的关联

         单向的N-N关联和1-N关联的持久化类完全一样，控制关系的一端需要增加一个set集合属性，被关联的持久化实例以集合的形式存在。

           N-N关联必须使用连接表，N-N关联与有连接表的1-N关联非常相似，只需要去掉<many-to-many.../>元素的unique="true"即可。其他的配置和1-N关联一样。

          由于与1-N关联非常相似，这里就不演示了。

         1.2双向N-N的关联

          对于双向的N-N关联，我们只需要转换为两个1-N关联模型即可。双向N-N关联两端都需要使用set集合属性，两端都增加对集合属性的访问。双向N-N同样必须使用连接表来建立两个实体之间的关联关系。

          以学生、老师为例:下面为两个实体的持久化类：

          Student

public class Student {
    private Integer id;
    private String name;
    private Set<Teacher> teachers;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Set<Teacher> getTeachers() {
        return teachers;
    }

    public void setTeachers(Set<Teacher> teachers) {
        this.teachers = teachers;
    }
}

         Teacher

public class Teacher {
    private Integer id;
    private String name;
    private Set<Student> students;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Set<Student> getStudents() {
        return students;
    }

    public void setStudents(Set<Student> students) {
        this.students = students;
    }

}

          双向N-N关联的映射文件需要使用<set.../>元素，用以映射集合属性。<set.../>属性还需要增加<key.../>子元素来映射外键列，同时还应该增加<many-to-many.../>子元素来映射关联实体类。两个映射文件如下：

          Student.hbm.xml

<hibernate-mapping package="com.hibernate.domain">
    <class name="Student" table="student">
        <id name="id" column="student_id">
            <generator class="native" />
        </id>
        
        <property name="name" column="student_name" />
        
        <!-- 映射N-N关联实体，两边的table应该以样 -->
        <set name="teachers" table="student_teacher">
            <!-- 映射关联的外键 列-->
            <key column="student_id" />
            <!-- 映射关联类属性 -->
            <many-to-many class="Teacher" column="teacher_id" />
        </set>
    </class>
</hibernate-mapping>

         Teacher.hbm.xml

<hibernate-mapping package="com.hibernate.domain">
    <class name="Teacher" table="teacher">
        <id name="id" column="teacher_id">
            <generator class="native" />
        </id>
        
        <property name="name" column="teacher_name" />
        
        <!-- 映射N-N关联实体，两边的table应该以样 -->
        <set name="students" table="student_teacher">
            <!-- 映射关联的外键 列-->
            <key column="teacher_id" />
            <!-- 映射关联类属性 -->
            <many-to-many class="Student" column="student_id"></many-to-many>
        </set>
    </class>
</hibernate-mapping>

          双向N-N关联的双边都需要指定连接表的表名，外键列的列名，所以两个<set.../>元素的table属性的值必须指定且一样。<set.../>元素的两个子元素:<key../>、<many-to-many.../>都必须指定column属性。<key.../>和<many-to-many.../>分别是指定本持久化类、关联类在连接表中的外键列。因此两边的<key.../>和<many-to-many.../>的column属性的值应该是交叉相等。

          通过下面的操作类来添加两个Student对象和两个Teacher对象

    static void add(){
        Session session = HibernateUtil.getSession();
        Transaction tx = session.beginTransaction();
        
        Teacher teacher1 = new Teacher();
        teacher1.setName("teacher1");
        
        Teacher teacher2 = new Teacher();
        teacher2.setName("teacher2");
        
        Student student1 = new Student();
        student1.setName("student1");
        
        Student student2 = new Student();
        student2.setName("student2");
        
        //建立两者之间的关系
        Set<Student> ss = new HashSet<Student>();
        ss.add(student1);
        ss.add(student2);
        
        teacher1.setStudents(ss);
        teacher2.setStudents(ss);
        
        session.save(teacher1);
        session.save(teacher2);
        session.save(student1);
        session.save(student2);
        
        tx.commit();
        session.close();
    }

          注意：这里只能由一边建立关联关系，就是说只能由Student对象建立与Teacher对象的关联或者由Teachert对象建立与Student对象的关联。否则将会出现主键重复错误