【SSH高速进阶】——Hibernate 多对多映射
说到多对多关系。印象最深刻的就是大学的选修课。一个学生能够选修多门课程,一门课程能够有多个学生选修,学生所选的每一门课程还有成绩。
这个场景的E-R图例如以下:
对于多对多的关系,我们一般会抽出一张中间表(连接表),来负责维护这两张表的多对多关系。比方上述关系应该生成的表结构为:
PO对象
Student.java
public class Student {
private int id;
private String name;
private Set<Course> courses;
//getter、setter
}
Course.java
public class Course {
private int id;
private String name;
private Set<Student> students;
//getter、setter
}
映射文件
Student.hbm.xml
<hibernate-mapping package="org.hibernate.test">
<class name="com.danny.hibernate.Student" table="t_student">
<id name="id">
<generator class="native" />
</id>
<property name="name" />
<set name="courses" table="t_signup">
<key column="student_id"></key>
<many-to-many class="com.bjpowernode.hibernate.Course" column="course_id"></many-to-many>
</set>
</class>
</hibernate-mapping>
Course.hbm.xml
<hibernate-mapping package="org.hibernate.test">
<class name="com.danny.hibernate.Course" table="t_course">
<id name="id">
<generator class="native" />
</id>
<property name="name" />
<set name="students" table="t_signup" inverse="true">
<key column="course_id"></key>
<many-to-many class="com.bjpowernode.hibernate.Student" column="student_id"></many-to-many>
</set>
</class>
</hibernate-mapping>
配置文件里的set相应于相应类中的集合,key是指向多的一方的外键,相应t_score表中的course_id。
执行代码执行的建表语句为:
alter table t_signup drop foreign key FK7DADC3438FFF3382
alter table t_signup drop foreign key FK7DADC3438CBEF332
drop table if exists t_course
drop table if exists t_signup
drop table if exists t_student
create table t_course (id integer not null auto_increment, name varchar(255), primary key (id))
create table t_signup (student_id integer not null, course_id integer not null, primary key (student_id, course_id))
create table t_student (id integer not null auto_increment, name varchar(255), primary key (id))
alter table t_signup add index FK7DADC3438FFF3382 (student_id), add constraint FK7DADC3438FFF3382 foreign key (student_id) references t_student (id)
alter table t_signup add index FK7DADC3438CBEF332 (course_id), add constraint FK7DADC3438CBEF332 foreign key (course_id) references t_course (id)
生成的表结构例如以下:
t_signup中生成了复合主键,student_id和course_id各自是指向t_student和t_course的外键。
插入測试
session.beginTransaction();
Course course1=new Course();
course1.setName("《心理应激微反应》");
session.save(course1);
Course course2=new Course();
course2.setName("《哈利·波特与遗传学》");
session.save(course2);
Course course3=new Course();
course3.setName("《三国杀攻略教程》");
session.save(course3);
Course course4=new Course();
course4.setName("《寄生虫与寄生虫病视频赞赏》");
session.save(course4);
Student student1=new Student();
Set courses1=new HashSet();
courses1.add(course1);
courses1.add(course2);
student1.setCourses(courses1);
student1.setName("小胡");
session.save(student1);
Student student2=new Student();
Set courses2=new HashSet();
courses2.add(course3);
courses2.add(course4);
student2.setCourses(courses2);
student2.setName("小玉");
session.save(student2);
Student student3=new Student();
Set courses3=new HashSet();
courses3.add(course1);
courses3.add(course2);
courses3.add(course3);
courses3.add(course4);
student3.setCourses(courses3);
student3.setName("小洋");
session.save(student3);
session.getTransaction().commit();
插入结果:
查询測试(上述配置是双向关联)
查询课程以及选修该课程的全部学生
session.beginTransaction();
Course course=(Course)session.load(Course.class, 1);
System.out.println("-"+course.getName());
for(Student s:course.getStudents()){
System.out.println("--"+s.getName());
}
session.getTransaction().commit();
查询结果:
Hibernate: select course0_.id as id2_0_, course0_.name as name2_0_ from t_course course0_ where course0_.id=?
-《心理应激微反应》
Hibernate: select students0_.course_id as course2_1_, students0_.student_id as student1_1_, student1_.id as id0_0_, student1_.name as name0_0_ from t_signup students0_ left outer join t_student student1_ on students0_.student_id=student1_.id where students0_.course_id=?
--小胡
--小洋
查询学生以及该学生所选修的全部课程
session.beginTransaction();
Course course=(Course)session.load(Course.class, 1);
System.out.println("-"+course.getName());
for(Student s:course.getStudents()){
System.out.println("--"+s.getName());
}
session.getTransaction().commit();
查询结果
Hibernate: select student0_.id as id0_0_, student0_.name as name0_0_ from t_student student0_ where student0_.id=?
-小胡
Hibernate: select courses0_.student_id as student1_1_, courses0_.course_id as course2_1_, course1_.id as id2_0_, course1_.name as name2_0_ from t_signup courses0_ left outer join t_course course1_ on courses0_.course_id=course1_.id where courses0_.student_id=?
--《心理应激微反应》
--《哈利·波特与遗传学》
细心的你应该早就发现了,每一个同学选修课程的成绩呢?
上述方法并不适合给多对多的关系加入额外的属性。那怎么办呢?能够用两个一对多关系来实现,即能够手动将中间表设计成一个实体,并为其配置映射关系。所以通常情况下,一个多对多关系也能够用两个一对多关系来实现。
【 转载请注明出处——胡玉洋《【SSH高速进阶】——Hibernate 多对多映射》】