JPA Many to One UniDirectional example

In this tutorial, I will show you how to implement JPA Many to One UniDirectional mapping with Hibernate in a Spring Boot example using @ManyToOne annotation. You'll know:

• How to configure Spring Data, JPA, Hibernate to work with Database
• How to define Data Models and Repository interfaces for JPA Many-to-One relationship
• Way to use Spring JPA to interact with Database for Many-to-One association
• Way to create Spring Rest Controller to process HTTP requests

Full Article: JPA Many to One UniDirectional example

JPA Many to One UniDirectional: Appropriate way to implement One To Many mapping

In a relational database, a One-to-Many relationship between table A and table B indicates that one row in table A links to many rows in table B, but one row in table B links to only one row in table A.

For example, you need to design data model for a Tutorial Blog in which One Tutorial has Many Comments. So this is a One-to-Many association.

You can map the child entities as a collection (List of Comments) in the parent object (Tutorial), and JPA/Hibernate provides the @OneToMany annotation for that case: only the parent-side defines the relationship. We call it unidirectional @OneToMany association.

Similarly, when only the child-side manage the relationship, we have unidirectional Many-to-One association with @ManyToOne annotation where the child (Comment) has an entity object reference to its parent entity (Tutorial) by mapping the Foreign Key column (tutorial_id).

The most appropriate way to implement JPA/Hibernate One To Many mapping is unidirectional Many-to-One association with @ManyToOne. You can read Vlad Mihalcea's article for more details.

JPA Many to One UniDirectional example

We're gonna create a Spring project from scratch, then we implement JPA/Hibernate One to Many Mapping with tutorials and comments table as following:

We also write Rest Apis to perform CRUD operations on the Comment entities.

These are APIs that we need to provide:

Methods	Urls	Actions
POST	/api/tutorials/:id/comments	create new Comment for a Tutorial
GET	/api/tutorials/:id/comments	retrieve all Comments of a Tutorial
GET	/api/comments/:id	retrieve a Comment by :id
PUT	/api/comments/:id	update a Comment by :id
DELETE	/api/comments/:id	delete a Comment by :id
DELETE	/api/tutorials/:id	delete a Tutorial (and its Comments) by :id
DELETE	/api/tutorials/:id/comments	delete all Comments of a Tutorial

Assume that we've had tutorials table like this:

Here are the example requests:

• Create new Comments: POST /api/tutorials/[:id]/comments

comments table after that:

• Retrieve all Comments of specific Tutorial: GET /api/tutorials/[:id]/comments

• Delete all Comments of specific Tutorial: DELETE /api/tutorials/[:id]/comments

Check the comment table, all Comments of Tutorial with id=2 were deleted:

• Delete a Tutorial: DELETE /api/tutorials/[:id]

All Comments of the Tutorial with id=3 were CASCADE deleted automatically.

Let's build our Spring Boot One to Many CRUD example.

Spring Boot Many to One example

Technology

• Java 8
• Spring Boot 2.6.2 (with Spring Web MVC, Spring Data JPA)
• PostgreSQL/MySQL
• Maven 3.8.1

Project Structure

Let me explain it briefly.

– Tutorial, Comment data model class correspond to entity and table tutorials, comments.
– TutorialRepository, CommentRepository are interfaces that extends JpaRepository for CRUD methods and custom finder methods. It will be autowired in TutorialController, CommentController.
– TutorialController, CommentController are RestControllers which has request mapping methods for RESTful CRUD API requests.
– Configuration for Spring Datasource, JPA & Hibernate in application.properties.
– pom.xml contains dependencies for Spring Boot and MySQL/PostgreSQL/H2 database.

– About exception package, to keep this post straightforward, I won't explain it. For more details, you can read following tutorial:
@RestControllerAdvice example in Spring Boot

For step by step and Github, please visit:
https://www.bezkoder.com/jpa-one-to-many/

You can apply this implementation in following tutorials:

• Spring JPA + H2 example
• Spring JPA + MySQL example
• Spring JPA + PostgreSQL example
• Spring JPA + Oracle example
• Spring JPA + SQL Server example

Further Reading

• Secure Spring Boot App with Spring Security & JWT Authentication
• Spring Data JPA Reference Documentation
• Spring Boot Pagination and Sorting example

If you want to add Pagination to this Spring project, you can find the instruction at:
Spring Boot Pagination & Filter example | Spring JPA, Pageable

To sort/order by multiple fields:
Spring Data JPA Sort/Order by multiple Columns | Spring Boot

Handle Exception for this Rest APIs is necessary:

• Spring Boot @ControllerAdvice & @ExceptionHandler example
• @RestControllerAdvice example in Spring Boot

Or way to write Unit Test for the JPA Repository:
Spring Boot Unit Test for JPA Repositiory with @DataJpaTest

You can also know:

• how to deploy this Spring Boot App on AWS (for free) with this tutorial.
• dockerize with Docker Compose: Spring Boot and MySQL example
• way to upload an Excel file and store the data in MySQL database with this post
• upload CSV file and store the data in MySQL with this post.