When it comes to the many-to-many relationship, the most impressive thing is the optional courses in universities.A student can take more than one course, a course can have more than one student, and each course he or she chooses has results.The E-R diagram for this scene is as follows:
For many-to-many relationships, we usually pull out an intermediate table (join table) to maintain the many-to-many relationship between the two tables. For example, the table structure that these relationships should generate is:
PO Object
Student.java
package entity; import java.util.Set; public class Students { private int id ; private String name; private Set<Course>courses; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Set<Course> getCourses() { return courses; } public void setCourses(Set<Course> courses) { this.courses = courses; } }
Course.java
package entity; import java.util.Set; public class Course { private int id; private String name; private Set<Students> students; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Set<Students> getStudents() { return students; } public void setStudents(Set<Students> students) { this.students = students; } }
Mapping File
Students.hnm.xml
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping> <class name ="entity.Students" table="t_students"> <id name="id"> <generator/> </id> <property name="name"/> <set name="Courses" table="t_singup"> <key column="student_id"/> <!-- Multiple Students id Corresponding pairs course_id -->
<many-to-many column="course_id"></many-to-many> </set> </class> </hibernate-mapping>
The set in the configuration file corresponds to the set in the corresponding class, the key is a foreign key to the more-directed side, corresponding to t_Course_in score tableID.
Course.hbm.xml
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping> <class name="entity.Course" table="t_course"> <id name="id"> <generator/> </id> <property name="name"/> <set name="students" table="t_signup" inverse="true"> <key column="course_id"/> <many-to-many column="student_id"/> </set> </class> </hibernate-mapping>
The table-building statements executed by running the code are:
drop table t_course drop table t_signup drop table t_students create table t_course (id integer primary key , name varchar(255)); create sequence course minvalue 1 --minimum value nomaxvalue --Do not set maximum start with 1 --Count from 1 increment by 1 --Add 1 at a time nocycle --Always accumulating,No cycle nocache --No Buffer create or replace trigger course_tg before insert on t_course for each row when(new.id is null) begin select course.nextval into:new.id from dual; end; create table t_signup (student_id integer , course_id integer ,constraint zhujian primary key (student_id, course_id)); drop sequence students; create table t_students (id integer , name varchar(255), primary key (id)); create sequence students minvalue 1 --minimum value nomaxvalue --Do not set maximum start with 1 --Count from 1 increment by 1 --Add 1 at a time nocycle --Always accumulating,No cycle nocache ;--No Buffer create or replace trigger students_tg before insert on t_students for each row when(new.id is null) begin select students.nextval into:new.id from dual; end; create index FK7DADC3438FFF3382 on t_signup(student_id); alter table t_signup add constraint FK7DADC3438FFF3382 foreign key(student_id) references t_students(id); create index FK7DADC3438CBEF332 on t_signup(course_id); alter table t_signup add constraint FK7DADC3438CBEF332 foreign key(course_id) references t_course(id);
The resulting table structure is as follows:
T_A composite primary key is generated in signup, student_id and course_id refers to t_students and t_Foreign key for course.
Insert Test
session.beginTransaction(); Course course1=new Course(); course1.setName("<Psychological Stress Microreactions"); session.save(course1); Course course2=new Course(); course2.setName("<Harry·Porter and Genetics"); session.save(course2); Course course3=new Course(); course3.setName("<Three Kingdoms Killing Strategy Tutorial"); session.save(course3); Course course4=new Course(); course4.setName("<Video Appreciation of Parasitic and Parasitic Diseases"); session.save(course4); Student student1=new Student(); Set courses1=new HashSet(); courses1.add(course1); courses1.add(course2); student1.setCourses(courses1); student1.setName("Xiao Hu"); session.save(student1); Student student2=new Student(); Set courses2=new HashSet(); courses2.add(course3); courses2.add(course4); student2.setCourses(courses2); student2.setName("Xiaoyu"); 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("ten-cent or twenty-cent silver coin"); session.save(student3); session.getTransaction().commit();
Insert results:
However, the above method is not suitable for adding additional attributes to many-to-many relationships. What should I do?You can use two one-to-many relationships, that is, you can manually design the intermediate table as an entity and configure mapping relationships for it, so in general, a many-to-many relationship can also be implemented with two one-to-many relationships.