JPA/Hibernate Many To Many Bidirectional Relationship Mapping

JPA/Hibernate Spring Boot

This post will walk you through the steps of mapping a JPA/Hibernate Many to Many bidirectional relationship with Spring Boot, Spring Data JPA, Lombok, MySQL and Docker.

JPA/Hibernate Entity Bidirectional Relationship Mapping Example Series with Spring Boot and MySQL
- JPA/Hibernate Composite Primary Key Example
- JPA/Hibernate One To One Bidirectional Relationship Mapping
- JPA/Hibernate One To One Shared Primary Key Bidirectional Relationship Mapping
- JPA/Hibernate One To Many Bidirectional Relationship Mapping
- JPA/Hibernate Many To Many Bidirectional Relationship Mapping
- JPA/Hibernate Many To Many Extra Columns Bidirectional Relationship Mapping
- Handling Circular Reference of JPA/Hibernate Entity Bidirectional Relationships with Jackson JsonIgnoreProperties
- JPA/Hibernate CRUD Example, Deleting Data with JPQL, CascadeType, and orphanRemoval
- Database Migration/Evolution Example with Flyway and JPA/Hibernate

What you'll need

Docker CE 18+

Project structure

Project dependencies

Many-To-Many Relationship

Many-to-many relationship refers to the relationship between two entities/tables A and B in which one element/row of A may be linked with many elements of B, and vice versa, one member of B may be linked to many elements of A.

In this example, the book and publisher tables have a many-to-many relationship. One book may be published by many publishers and one publisher may publish many books.

book_publisher is a join table of book and publisher.

book_publisher.book_id is a foreign key references to book.id, book_publisher.publisher_id is a foreign key references to publisher.id.

book_id and publisher_id is also a composite primary key of book_publisher.

Try this example if the join table has extra columns beside two foreign keys
JPA/Hibernate Many To Many Extra Columns Example of Bidirectional Relationship Mapping

Define JPA Entities

JPA Entity is defined with @Entity annotation, represent a table in your database.

@Id declares the entity identifier.

@Column maps the entity's field with the table's column. If @Column is omitted, the field name of the entity will be used as column name by default.

@ManyToMany defines a many-to-many relationship between 2 entities. mappedBy indicates the entity is the inverse of the relationship.

@JoinTable defines the join table of 2 associated entities. If the JoinTable annotation is missing, the default values of the annotation elements apply. The name of the join table is assumed to be the table names of the associated primary tables concatenated together (owning side first) using an underscore.

Spring Data JPA Repository

Spring Data JPA contains some built-in Repository abstracting common functions based on EntityManager to work with database such as findAll, findById, save, delete, deleteById. All we need for this example is extends JpaRepository.

Application Properties

spring.jpa.hibernate.ddl-auto=create allows JPA/Hibernate auto create database and table schema for you.

On production environment, you may like to disable the DDL Auto feature by using spring.jpa.hibernate.ddl-auto=validate or spring.jpa.hibernate.ddl-auto=none (default). Check out this example as one of the approaches
Database Migration/Evolution Example with Flyway and JPA/Hibernate

Creating data with JPA/Hibernate

Thanks to CascadeType.ALL, associated entity Publisher will be saved at the same time with Book without the need of calling its save function explicitly.

Testing time

Prepare Dockerfile for Java/Spring Boot application

Start your application and infrastructure via Docker Compose. Make sure your local Docker is running and your bash console is at the springboot-examples project root directory.

docker-compose -f docker-compose-jpa.yaml up --renew-anon-volumes many-to-many

Access to MySQL Server docker container by issuing below bash command and key in hellokoding on Enter password:

docker exec -it hk-mysql mysql -p

Query the schema and data created by JPA/Hibernate based on your mapping

That's it! Thanks for joining and have a happy coding. You can find the full source code as below

Source code

https://github.com/hellokoding/springboot-examples/tree/master/jpa-hibernate/many-to-many

You may also like
- Handling Circular Reference of JPA/Hibernate Entity Bidirectional Relationships with Jackson JsonIgnoreProperties
- JPA/Hibernate CRUD Example, Deleting Data with JPQL, CascadeType, and orphanRemoval

JPA/Hibernate Spring Boot