Hibernate：多对多

Hibernate处理多对多的情况，在操作和性能方面都不太理想，所以多对多的映射使用较少，实际使用中最好转换成一对多的对象模型：Hibernate会为我们创建中间关联表，转换成两个一对多。

POJO对象：

import java.util.Set;

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;
	}
}

import java.util.Set;

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;
	}
	
}

映射文件：

<?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 package="cn.itcast.hibernate.domain">

	<class name="Student" table="student">
		<id name="id">
			<generator class="native" />
		</id>
		<property name="name"/>
		<set name="teachers" table="teacher_student">
			<key column="student_id"/>
			<many-to-many class="Teacher" column="teacher_id"/>
		</set>
	</class>
</hibernate-mapping>

因为由中间表，所以现在要找一个教师的所有学生，应该根据映射文件，首先找到此教师的id，然后到中间表中根据此教师id找到它对应的学生id，然后拿着此学生的id到学生表中找到一条学生信息。所以配置的选项：
1 要找此教师的学生，应该找到学生集合的属性名 students,依据<set name="students" table="teacher_student">
2 在里面同时指定了要找的中间表，要知道中间表中教师的id对应的字段
<key column="teacher_id"/>
3 在此中间表中找到一条此教师id:teacher_id对应的学生的id：student_id;
4 拿着此学生id到学生表中找到一条符合条件的记录（在对象模型中也就是一个学生对象）

<?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 package="cn.itcast.hibernate.domain">

	<class name="Teacher" table="teacher">
		<id name="id" unsaved-value="-1">
			<generator class="native" />
		</id>
		<property name="name"/>
		<set name="students" table="teacher_student">
		  <key column="teacher_id"/>
		  <many-to-many class="Student" column="student_id"/>	
		</set>
	</class>
</hibernate-mapping>

测试类：

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

import org.hibernate.Session;
import org.hibernate.Transaction;

import cn.itcast.hibernate.HibernateUtil;
import cn.itcast.hibernate.domain.Student;
import cn.itcast.hibernate.domain.Teacher;

public class ManyToManyTest {

	public static void main(String[] args) {
		
		Session session = HibernateUtil.getSession();
		Transaction tx = session.beginTransaction();
		
		Set<Student> students = new HashSet<Student>();
		Set<Teacher> teachers = new HashSet<Teacher>();		
		
		Student student1 = new Student();
		student1.setName("学生1");
		Student student2 = new Student();
		student2.setName("学生2");
		
		Teacher teacher1 = new Teacher();
		teacher1.setName("教师1");
		Teacher teacher2 = new Teacher();
		teacher2.setName("教师2");
		
		students.add(student1);
		students.add(student2);
		
		teachers.add(teacher1);
		teachers.add(teacher2);
		
		student1.setTeachers(teachers);
		student2.setTeachers(teachers);
		
		//下面两条千万不能要！因为中间表采用的是联合主键，所以只要一端加入即可
		//如果加入两次，就会出现主键冲突
		//teacher1.setStudents(students);
		//teacher2.setStudents(students);
		
		session.save(student1);
		session.save(student2);
		session.save(teacher1);
		session.save(teacher2);
		
		tx.commit();
		session.close();
		
	}

}

注意，多对多的插入实际上是在中间表中加数据的，而且中间表的表结构，是以两个对象的主键组合作为主键，也就是联合主键，也就是说a,b与b,a是认为重复的！！所以在添加的时候只在一方添加即可！！

可以看到表结构的主键为联合主键，所以要是在两端都添加会有冲突！
t1.setStudents(stuSet);
t2.setStudents(stuSet);

//s1.setTeachers(teaSet);
//s2.setTeachers(teaSet);
就是说只能执行上面两句或者下面两句！！要是全部执行就构成了重复



多对多的查询：

		Student student = (Student)session.get(Student.class, 1);
		Set<Teacher> teachers = student.getTeachers();
		
		for(Teacher teacher : teachers){
			System.out.println(teacher.getName());
		}

Hibernate: 
    select
        student0_.id as id6_0_,
        student0_.name as name6_0_ 
    from
        student student0_ 
    where
        student0_.id=?
Hibernate: 
    select
        teachers0_.student_id as student2_1_,
        teachers0_.teacher_id as teacher1_1_,
        teacher1_.id as id4_0_,
        teacher1_.name as name4_0_ 
    from
        teacher_student teachers0_ 
    left outer join
        teacher teacher1_ 
            on teachers0_.teacher_id=teacher1_.id 
    where
        teachers0_.student_id=?

可以看到到中间表去查一个教师有多少个学生！！！所以查询需要三个表，效率低下，而且如果关联多个对象，比如一个热帖回复有1万个，这样，利用getSet()就会得到一万多条数据，所以尽量少用。