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https://github.com/nfriaa/hibernate-tutorial6

Hibernate tutorial 6 : "Many To Many" association
https://github.com/nfriaa/hibernate-tutorial6

annotations hibernate hibernate5 java many-to-many maven mysql

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Hibernate tutorial 6 : "Many To Many" association

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# hibernate-tutorial6

Hibernate tutorial 6 : "Many To Many" association



[![contributions welcome](https://img.shields.io/badge/contributions-welcome-orange.svg?style=flat)](https://github.com/nfriaa/hibernate-tutorial6/issues) [![Travis](https://img.shields.io/travis/rust-lang/rust.svg)](https://github.com/nfriaa/hibernate-tutorial6) [![license](https://img.shields.io/github/license/mashape/apistatus.svg)](https://github.com/nfriaa/hibernate-tutorial6/blob/master/LICENSE)



## Description

A sample code to learn how to map **"Many To Many"** relationship between two entities using the Hibernate ORM.

* JavaSE 8

* Hibernate 5 / Annotations

* Hibernate **"Many To Many"** association

* Maven 4

* MySQL 5



## 1. Database

Create only database, don't create tables (tables will be created by Hibernate)

* database name : **persist_db**

```sql

CREATE DATABASE `persist_db` /*!40100 DEFAULT CHARACTER SET utf8 */

```



## 2. Maven "pom.xml" dependencies

```xml



    

    

        mysql

        mysql-connector-java

        6.0.6

    

    

    

        org.hibernate

        hibernate-core

        5.2.11.Final

    



```



## 3. Hibernate configuration file "hibernate.cfg.xml"

```xml



    

        

        org.hibernate.dialect.MySQL5InnoDBDialect

        jdbc:mysql://localhost:3306/persist_db?useTimezone=true&serverTimezone=UTC

        root

        



        

        true

        false

        create



        

        

        

    



```

* hibernate.hbm2ddl.auto : "create" => creates the schema necessary for defined entities, destroying any previous data

* Entities : map the two entities in this XML config file (Product and Category)



## 4. "Many To Many" association

`Destination entity : Product.java`

```java

import java.util.ArrayList;

import java.util.List;

import javax.persistence.CascadeType;

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

import javax.persistence.JoinColumn;

import javax.persistence.JoinTable;

import javax.persistence.ManyToMany;

import javax.persistence.Table;



@Entity

@Table(name = "product")

public class Product {



    @Id

    @GeneratedValue(strategy = GenerationType.IDENTITY)

    @Column(name = "id", nullable = false)

    private int id;



    @Column(name = "name", length = 255, nullable = true)

    private String name;



    @Column(name = "price", nullable = true)

    private int price;



    @ManyToMany(cascade = CascadeType.ALL)

    @JoinTable(name = "product_category",

            joinColumns = {

                @JoinColumn(name = "product_id")},

            inverseJoinColumns = {

                @JoinColumn(name = "category_id")})

    private List categories = new ArrayList<>();



    // Getters and Setters here...

}

```



`Source entity : Category.java`

```java

import java.util.ArrayList;

import java.util.List;

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.FetchType;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

import javax.persistence.ManyToMany;

import javax.persistence.Table;



@Entity

@Table(name = "category")

public class Category {



    @Id

    @GeneratedValue(strategy = GenerationType.IDENTITY)

    @Column(name = "id", nullable = false)

    private int id;



    @Column(name = "name", length = 255, nullable = true)

    private String name;



    // uncomment this to have bidirectionnel mode

    // Bidirectionnel "Many To Many" :

    //    @ManyToMany(fetch = FetchType.LAZY, mappedBy = "categories")

    //    private List products = new ArrayList<>();

    //    public List getProducts() {

    //        return products;

    //    }

    //

    //    public void setProducts(List products) {

    //        this.products = products;

    //    }



    // Getters and Setters here...

}

```

* @OManyToMany : the **Many To Many** association always uses an intermediate join table to store the association that joins two entities

* There are two types of **Many To Many** association : 

    * Unidirectional : only source entity has a relationship field that refers to the target entity (our current example is unidirectional)

    * Bidirectional : each entity (i.e. source and target) has a relationship field that refers to each other



## 5. Main Class "Application.java"

* create main class to test the code

* example :

```java

    // new category

    Category category_a = new Category();

    category_a.setName("Cat a");

    session.save(category_a);



    // new category

    Category category_b = new Category();

    category_b.setName("Cat b");

    session.save(category_b);



    // new product

    Product product_x = new Product();

    product_x.setName("Prod x");

    product_x.setPrice(456);

    product_x.getCategories().add(category_b);

    session.save(product_x);



    // new product

    Product product_y = new Product();

    product_y.setName("Prod y");

    product_y.setPrice(123);

    product_y.getCategories().add(category_a);

    session.save(product_y);



    // new product

    Product product_z = new Product();

    product_z.setName("Prod z");

    product_z.setPrice(789);

    product_z.getCategories().add(category_a);

    session.save(product_z);

```