This tutorial will walk you through the steps of mapping a JPA and Hibernate One To Many bidirectional entity relationships example with with Spring Boot, Spring Data JPA, Lombok, MySQL and Docker

What you'll need

  • JDK 8+ or OpenJDK 8+
  • Maven 3+
  • MySQL Server 5+ or Docker CE 18+

Init project structure and dependencies

Project structure

├── src
│   └── main
│       ├── java
│       │   └── com
│       │       └── hellokoding
│       │           ├── jpa
│       │           │   ├── book
│       │           │   │   ├──
│       │           │   │   ├──
│       │           │   │   ├──
│       │           │   │   └──
│       │           │   └──
│       │           └── springboot
│       └── resources
│           └──
├── Dockerfile
├── docker-compose.yml
└── pom.xml

Project dependencies

Define JPA Entities and Repositories

One-To-Many Relationship

One-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, but a member of B is linked to only one element of A.

In this example, the book_category and book tables have a one-to-many relationship. One category may be linked with many books but one book is linked to only one category.

book.book_category_id is a foreign key references to

JPA and Hibernate 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.

@OneToMany and @ManyToOne defines a one-to-many and many-to-one relationship between 2 entities. @JoinColumn indicates the entity is the owner of the relationship and the corresponding table has a column with a foreign key to the referenced table. mappedBy indicates the entity is the inverse of the relationship.

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.

Define properties and creating data

Application Properties

hk-mysql refers to Docker Compose service defined in the below docker-compose.yml file

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 and Hibernate

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

Run the example

Run with Docker

Prepare Dockerfile for Java/Spring Boot application and docker-compose.yml for MySQL Server

Type the below command at the project root directory, make sure your local Docker is running

docker-compose up  

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

Run with JDK/OpenJDK, Maven and MySQL Server local

Update hk-mysql on to localhost and type the below command at the project root directory

mvn clean spring-boot:run  

Source code

See also