Hibernate
Many to many relationship in hibernate
1. Introduction
A many-to-many mapping represents a relationship between two entities, where each entity can be associated with multiple instances of the other entity. In a many-to-many relationship between two tables, there will be an intermediate table (common table or join table) that stores the relationship between the two entities.
Hibernate create a junction table (intermediary table) while establishing many-to-many relationship between two entities & this junction table store the relationship between two entities. You can create a one-to-many mapping in Hibernate using the @ManyToMany annotation.
2: Example: Lets take two entity class i.e Employee & Project. Here one employee can work on many projects & one project assigned to many employee
As we can see Employee1 & Employee2 one has assigend for 5 projects & every projects has assinged to bote. it shows the many-to-many relationship.
Lets write code for this.
Create a Maven Project as below groupId is com.hiber.manytomany & artifactId is ManyToMany
Create two entity class Emp.java & Project.java for many-to-many mapping.
Create a database in mysql( I am using mysql here) by the name of manytomanydb
Create a main class called MappingDemo.java
pom.xml
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.hiber.manytomany</groupId>
<artifactId>ManyToMany</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>
<name>ManyToMany</name>
<url>http://maven.apache.org</url>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
</properties>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>3.8.1</version>
<scope>test</scope>
</dependency>
<!-- https://mvnrepository.com/artifact/org.hibernate/hibernate-core -->
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
<version>5.6.6.Final</version>
</dependency>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.30</version>
</dependency>
</dependencies>
</project>
hibernate.cfg.xml
<?xml version = "1.0" encoding = "utf-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="connection.url">jdbc:mysql://localhost:3306/manytomanydb</property>
<property name="connection.username">root</property>
<property name="connection.password">root</property>
<property name = "dialect">org.hibernate.dialect.MySQL5Dialect</property>
<property name="hbm2ddl.auto">create</property>
<property name="show_sql">true</property>
<mapping class="com.hiber.manytomany.Project"/>
<mapping class="com.hiber.manytomany.Emp"/>
</session-factory>
</hibernate-configuration>Emp.java
package com.hiber.manytomany;
import java.util.List;
import javax.persistence.*;
@Entity
public class Emp {
@Id
private int eid;
private String name;
@ManyToMany
@JoinTable(
name="emp_project",joinColumns = {@JoinColumn(name="eid")},
inverseJoinColumns = {@JoinColumn(name="pid")}
)
private List<Project> projects;
public int getEid() {
return eid;
}
public void setEid(int eid) {
this.eid = eid;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Project> getProjects() {
return projects;
}
public void setProjects(List<Project> projects) {
this.projects = projects;
}
public Emp() {
super();
// TODO Auto-generated constructor stub
}
public Emp(int eid, String name, List<Project> projects) {
super();
this.eid = eid;
this.name = name;
this.projects = projects;
}
}
Project.java
package com.hiber.manytomany;
import java.util.List;
import javax.persistence.Column;
import javax.persistence.*;
import javax.persistence.ManyToMany;
@Entity
public class Project {
@Id
private int pid;
@Column(name="project_name")
private String projectName;
@ManyToMany(mappedBy = "projects")
private List<Emp> emps;
public Project() {
super();
// TODO Auto-generated constructor stub
}
public Project(int pid, String projectName, List<Emp> emps) {
super();
this.pid = pid;
this.projectName = projectName;
this.emps = emps;
}
public int getPid() {
return pid;
}
public void setPid(int pid) {
this.pid = pid;
}
public String getProjectName() {
return projectName;
}
public void setProjectName(String projectName) {
this.projectName = projectName;
}
public List<Emp> getEmps() {
return emps;
}
public void setEmps(List<Emp> emps) {
this.emps = emps;
}
}
Output
Database Tables
mysql> use manytomanydb;
Database changed
mysql> show tables;
+------------------------+
| Tables_in_manytomanydb |
+------------------------+
| emp |
| emp_learn |
| emp_project |
| project |
+------------------------+
4 rows in set (0.00 sec)
mysql> select * from emp;
+-----+-------+
| eid | name |
+-----+-------+
| 34 | Ram |
| 35 | Shyam |
+-----+-------+
2 rows in set (0.02 sec)
mysql> select * from project;
+-------+--------------------------+
| pid | project_name |
+-------+--------------------------+
| 12121 | Library Managment System |
| 14214 | Chatbot |
+-------+--------------------------+
2 rows in set (0.01 sec)
mysql> select * from emp_project;
+-----+-------+
| eid | pid |
+-----+-------+
| 34 | 12121 |
| 34 | 14214 |
+-----+-------+
2 rows in set (0.01 sec)jointable
@JoinTable(
name="emp_project",joinColumns = {@JoinColumn(name="eid")},
inverseJoinColumns = {@JoinColumn(name="pid")}
)
Here intermediate table is emp_project having one primary key (eid)
& foreign key(pid) as shown in the above code. we can change
both coumn name using annotation.
1:To change the Emp table primary key use @JoinColumn(name="eid")
2:To change the Project table primary key ( here this is foreign key
in emp_project table)use inverseJoinColumns = {@JoinColumn(name="pid")