Tien Nguyen

Posted on Aug 7, 2023 • Edited on Aug 9, 2023 • Originally published at bezkoder.com

JPA @ManyToOne example in Spring Boot

#webdev #java #todayilearned #springboot

In this tutorial, I will show you how to implement Spring Data JPA Many-to-One example in Spring Boot for One-To-Many mapping 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 One-To-Many relationship using @ManyToOne
Way to use Spring JPA to interact with Database for Many-to-One association
Way to create Spring Rest Controller to process HTTP requests

JPA @ManyToOne is appropriate way for One To Many mapping in Spring

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.

For tutorial, please visit: JPA One To Many Unidirectional example.

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 because:

With @OneToMany, we need to declare a collection (Comments) inside parent class (Tutorial), we cannot limit the size of that collection, for example, in case of pagination.
With @ManyToOne, you can modify the Repository:
- to work with Pagination
- or to sort/order by multiple fields

JPA ManyToOne example

We're gonna create a Spring project from scratch, then we implement JPA/Hibernate Many to One 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 @ManyToOne CRUD example.

Spring Boot ManyToOne example

Technology

Java 17 / 11 / 8
Spring Boot 3 / 2 (with Spring Web MVC, Spring Data JPA)
H2/PostgreSQL/MySQL
Maven

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

Create & Setup Spring Boot project

Use Spring web tool or your development tool (Spring Tool Suite, Eclipse, Intellij) to create a Spring Boot project.

Then open pom.xml and add these dependencies:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

We also need to add one more dependency.

If you want to use MySQL:

<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <scope>runtime</scope>
</dependency>

or PostgreSQL:

<dependency>
    <groupId>org.postgresql</groupId>
    <artifactId>postgresql</artifactId>
    <scope>runtime</scope>
</dependency>

or H2 (embedded database):

<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <scope>runtime</scope>
</dependency>

Configure Spring Datasource, JPA, Hibernate

Under src/main/resources folder, open application.properties and write these lines.

For MySQL:

spring.datasource.url=jdbc:mysql://localhost:3306/testdb?useSSL=false
spring.datasource.username=root
spring.datasource.password=123456

spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQLDialect

# Hibernate ddl auto (create, create-drop, validate, update)
spring.jpa.hibernate.ddl-auto=update

For PostgreSQL:

spring.datasource.url=jdbc:postgresql://localhost:5432/testdb
spring.datasource.username=postgres
spring.datasource.password=123

spring.jpa.properties.hibernate.jdbc.lob.non_contextual_creation=true
spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.PostgreSQLDialect

# Hibernate ddl auto (create, create-drop, validate, update)
spring.jpa.hibernate.ddl-auto=update

spring.datasource.username & spring.datasource.password properties are the same as your database installation.
Spring Boot uses Hibernate for JPA implementation, we configure MySQLDialect for MySQL or PostgreSQLDialect for PostgreSQL
spring.jpa.hibernate.ddl-auto is used for database initialization. We set the value to update value so that a table will be created in the database automatically corresponding to defined data model. Any change to the model will also trigger an update to the table. For production, this property should be validate.

For H2 database:

spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.driverClassName=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=

spring.jpa.show-sql=true
spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.H2Dialect
spring.jpa.hibernate.ddl-auto=update

spring.h2.console.enabled=true
# default path: h2-console
spring.h2.console.path=/h2-ui

spring.datasource.url: jdbc:h2:mem:[database-name] for In-memory database and jdbc:h2:file:[path/database-name] for disk-based database.
We configure H2Dialect for H2 Database
spring.h2.console.enabled=true tells the Spring to start H2 Database administration tool and you can access this tool on the browser: http://localhost:8080/h2-console.
spring.h2.console.path=/h2-ui is for H2 console's url, so the default url http://localhost:8080/h2-console will change to http://localhost:8080/h2-ui.

Define Data Model for JPA ManyToOne mapping

In model package, we define Tutorial and Comment class.

Tutorial has four fields: id, title, description, published.

model/Tutorial.java

package com.bezkoder.spring.hibernate.onetomany.model;

import jakarta.persistence.*;

@Entity
@Table(name = "tutorials")
public class Tutorial {

  @Id
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "tutorial_generator")
  private long id;

  @Column(name = "title")
  private String title;

  @Column(name = "description")
  private String description;

  @Column(name = "published")
  private boolean published;

  public Tutorial() {

  }

  public Tutorial(String title, String description, boolean published) {
    this.title = title;
    this.description = description;
    this.published = published;
  }

  // getters and setters
}

@Entity annotation indicates that the class is a persistent Java class.
@Table annotation provides the table that maps this entity.
@Id annotation is for the primary key.
@GeneratedValue annotation is used to define generation strategy for the primary key. GenerationType.SEQUENCE means using database sequence to generate unique values.
We also indicate the name of the primary key generator. If you don't give it the name, id value will be generated with hibernate_sequence table (supplied by persistence provider, for all entities) by default.
@Column annotation is used to define the column in database that maps annotated field.

The Comment class has the @ManyToOne annotation for many-to-one relationship with the Tutorial entity. optional element is set to false for non-null relationship.

model/Comment.java

package com.bezkoder.spring.hibernate.onetomany.model;

import jakarta.persistence.*;

import org.hibernate.annotations.OnDelete;
import org.hibernate.annotations.OnDeleteAction;

import com.fasterxml.jackson.annotation.JsonIgnore;

@Entity
@Table(name = "comments")
public class Comment {
  @Id
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "comment_generator")
  private Long id;

  @Lob
  private String content;

  @ManyToOne(fetch = FetchType.LAZY, optional = false)
  @JoinColumn(name = "tutorial_id", nullable = false)
  @OnDelete(action = OnDeleteAction.CASCADE)
  @JsonIgnore
  private Tutorial tutorial;

  // getters and setters
}

We also use the @JoinColumn annotation to specify the foreign key column (tutorial_id). If you don't provide the JoinColumn name, the name will be set automatically.

@JsonIgnore is used to ignore the logical property used in serialization and deserialization.

We also implement cascade delete capabilities of the foreign-key with @OnDelete(action = OnDeleteAction.CASCADE).

We set the @ManyToOne with FetchType.LAZY for fetch type:

By default, the @ManyToOne association uses FetchType.EAGER for fetch type but it is bad for performance:

public class Comment {
  ...

  @ManyToOne(fetch = FetchType.EAGER, optional = false)
  @JoinColumn(name = "tutorial_id", nullable = false)
  @OnDelete(action = OnDeleteAction.CASCADE)
  private Tutorial tutorial;

  ...
}

Create Repository Interfaces for ManyToOne mapping

Let's create a repository to interact with database.
In repository package, create TutorialRepository and CommentRepository interfaces that extend JpaRepository.

repository/TutorialRepository.java

package com.bezkoder.spring.hibernate.onetomany.repository;

import java.util.List;

import org.springframework.data.jpa.repository.JpaRepository;

import com.bezkoder.spring.hibernate.onetomany.model.Tutorial;

public interface TutorialRepository extends JpaRepository<Tutorial, Long> {
  List<Tutorial> findByPublished(boolean published);

  List<Tutorial> findByTitleContaining(String title);
}

repository/CommentRepository.java

package com.bezkoder.spring.hibernate.onetomany.repository;

import java.util.List;

import jakarta.transaction.Transactional;

import org.springframework.data.jpa.repository.JpaRepository;

import com.bezkoder.spring.hibernate.onetomany.model.Comment;

public interface CommentRepository extends JpaRepository<Comment, Long> {
  List<Comment> findByTutorialId(Long postId);

  @Transactional
  void deleteByTutorialId(long tutorialId);
}

Now we can use JpaRepository's methods: save(), findOne(), findById(), findAll(), count(), delete(), deleteById()... without implementing these methods.

We also define custom finder methods:

findByPublished(): returns all Tutorials with published having value as input published.
findByTitleContaining(): returns all Tutorials which title contains input title.
findByTutorialId(): returns all Comments of a Tutorial specified by tutorialId.
deleteByTutorialId(): deletes all Comments of a Tutorial specified by tutorialId.

The implementation is plugged in by Spring Data JPA automatically.

Now we can see the pros of @ManyToOne annotation.

With @OneToMany, we need to declare a collection inside parent class, we cannot limit the size of that collection, for example, in case of pagination.
With @ManyToOne, you can modify Repository:
- to work with Pagination, the instruction can be found at: Spring Boot Pagination & Filter example | Spring JPA, Pageable
- or to sort/order by multiple fields: Spring Data JPA Sort/Order by multiple Columns | Spring Boot
Please notice that above tutorials are for TutorialRepository, you need to apply the same way for CommentRepository .

More Derived queries at:
JPA Repository query example in Spring Boot

Custom query with @Query annotation:
Spring JPA @Query example: Custom query in Spring Boot

You also find way to write Unit Test for this JPA Repository at:
Spring Boot Unit Test for JPA Repository with @DataJpaTest

Create Spring Rest APIs Controller

Finally, we create controller that provides APIs for CRUD operations: creating, retrieving, updating, deleting and finding Tutorials and Comments.

controller/TutorialController.java

package com.bezkoder.spring.hibernate.onetomany.controller;

import java.util.ArrayList;
import java.util.List;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.CrossOrigin;
import org.springframework.web.bind.annotation.DeleteMapping;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.PutMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

import com.bezkoder.spring.hibernate.onetomany.exception.ResourceNotFoundException;
import com.bezkoder.spring.hibernate.onetomany.model.Tutorial;
import com.bezkoder.spring.hibernate.onetomany.repository.TutorialRepository;

@CrossOrigin(origins = "http://localhost:8081")
@RestController
@RequestMapping("/api")
public class TutorialController {

  @Autowired
  TutorialRepository tutorialRepository;

  @GetMapping("/tutorials")
  public ResponseEntity<List<Tutorial>> getAllTutorials(@RequestParam(required = false) String title) {
    List<Tutorial> tutorials = new ArrayList<Tutorial>();

    if (title == null)
      tutorialRepository.findAll().forEach(tutorials::add);
    else
      tutorialRepository.findByTitleContaining(title).forEach(tutorials::add);

    if (tutorials.isEmpty()) {
      return new ResponseEntity<>(HttpStatus.NO_CONTENT);
    }

    return new ResponseEntity<>(tutorials, HttpStatus.OK);
  }

  @GetMapping("/tutorials/{id}")
  public ResponseEntity<Tutorial> getTutorialById(@PathVariable("id") long id) {
    Tutorial tutorial = tutorialRepository.findById(id)
        .orElseThrow(() -> new ResourceNotFoundException("Not found Tutorial with id = " + id));

    return new ResponseEntity<>(tutorial, HttpStatus.OK);
  }

  @PostMapping("/tutorials")
  public ResponseEntity<Tutorial> createTutorial(@RequestBody Tutorial tutorial) {
    Tutorial _tutorial = tutorialRepository.save(new Tutorial(tutorial.getTitle(), tutorial.getDescription(), true));
    return new ResponseEntity<>(_tutorial, HttpStatus.CREATED);
  }

  @PutMapping("/tutorials/{id}")
  public ResponseEntity<Tutorial> updateTutorial(@PathVariable("id") long id, @RequestBody Tutorial tutorial) {
    Tutorial _tutorial = tutorialRepository.findById(id)
        .orElseThrow(() -> new ResourceNotFoundException("Not found Tutorial with id = " + id));

    _tutorial.setTitle(tutorial.getTitle());
    _tutorial.setDescription(tutorial.getDescription());
    _tutorial.setPublished(tutorial.isPublished());

    return new ResponseEntity<>(tutorialRepository.save(_tutorial), HttpStatus.OK);
  }

  @DeleteMapping("/tutorials/{id}")
  public ResponseEntity<HttpStatus> deleteTutorial(@PathVariable("id") long id) {
    tutorialRepository.deleteById(id);

    return new ResponseEntity<>(HttpStatus.NO_CONTENT);
  }

  @DeleteMapping("/tutorials")
  public ResponseEntity<HttpStatus> deleteAllTutorials() {
    tutorialRepository.deleteAll();

    return new ResponseEntity<>(HttpStatus.NO_CONTENT);
  }

  @GetMapping("/tutorials/published")
  public ResponseEntity<List<Tutorial>> findByPublished() {
    List<Tutorial> tutorials = tutorialRepository.findByPublished(true);

    if (tutorials.isEmpty()) {
      return new ResponseEntity<>(HttpStatus.NO_CONTENT);
    }

    return new ResponseEntity<>(tutorials, HttpStatus.OK);
  }
}

controller/CommentController.java

package com.bezkoder.spring.hibernate.onetomany.controller;

import java.util.List;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.CrossOrigin;
import org.springframework.web.bind.annotation.DeleteMapping;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.PutMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import com.bezkoder.spring.hibernate.onetomany.exception.ResourceNotFoundException;
import com.bezkoder.spring.hibernate.onetomany.model.Comment;
import com.bezkoder.spring.hibernate.onetomany.repository.CommentRepository;
import com.bezkoder.spring.hibernate.onetomany.repository.TutorialRepository;

@CrossOrigin(origins = "http://localhost:8081")
@RestController
@RequestMapping("/api")
public class CommentController {

  @Autowired
  private TutorialRepository tutorialRepository;

  @Autowired
  private CommentRepository commentRepository;

  @GetMapping("/tutorials/{tutorialId}/comments")
  public ResponseEntity<List<Comment>> getAllCommentsByTutorialId(@PathVariable(value = "tutorialId") Long tutorialId) {
    if (!tutorialRepository.existsById(tutorialId)) {
      throw new ResourceNotFoundException("Not found Tutorial with id = " + tutorialId);
    }

    List<Comment> comments = commentRepository.findByTutorialId(tutorialId);
    return new ResponseEntity<>(comments, HttpStatus.OK);
  }

  @GetMapping("/comments/{id}")
  public ResponseEntity<Comment> getCommentsByTutorialId(@PathVariable(value = "id") Long id) {
    Comment comment = commentRepository.findById(id)
        .orElseThrow(() -> new ResourceNotFoundException("Not found Comment with id = " + id));

    return new ResponseEntity<>(comment, HttpStatus.OK);
  }

  @PostMapping("/tutorials/{tutorialId}/comments")
  public ResponseEntity<Comment> createComment(@PathVariable(value = "tutorialId") Long tutorialId,
      @RequestBody Comment commentRequest) {
    Comment comment = tutorialRepository.findById(tutorialId).map(tutorial -> {
      commentRequest.setTutorial(tutorial);
      return commentRepository.save(commentRequest);
    }).orElseThrow(() -> new ResourceNotFoundException("Not found Tutorial with id = " + tutorialId));

    return new ResponseEntity<>(comment, HttpStatus.CREATED);
  }

  @PutMapping("/comments/{id}")
  public ResponseEntity<Comment> updateComment(@PathVariable("id") long id, @RequestBody Comment commentRequest) {
    Comment comment = commentRepository.findById(id)
        .orElseThrow(() -> new ResourceNotFoundException("CommentId " + id + "not found"));

    comment.setContent(commentRequest.getContent());

    return new ResponseEntity<>(commentRepository.save(comment), HttpStatus.OK);
  }

  @DeleteMapping("/comments/{id}")
  public ResponseEntity<HttpStatus> deleteComment(@PathVariable("id") long id) {
    commentRepository.deleteById(id);

    return new ResponseEntity<>(HttpStatus.NO_CONTENT);
  }

  @DeleteMapping("/tutorials/{tutorialId}/comments")
  public ResponseEntity<List<Comment>> deleteAllCommentsOfTutorial(@PathVariable(value = "tutorialId") Long tutorialId) {
    if (!tutorialRepository.existsById(tutorialId)) {
      throw new ResourceNotFoundException("Not found Tutorial with id = " + tutorialId);
    }

    commentRepository.deleteByTutorialId(tutorialId);
    return new ResponseEntity<>(HttpStatus.NO_CONTENT);
  }
}

Conclusion

Today we've built a Spring Boot CRUD example using Spring Data JPA, Hibernate Many To One relationship with MySQL/PostgreSQL/embedded database (H2).

We also see that @ManyToOne annotation is the most appropriate way for implementing JPA One To Many Mapping, and JpaRepository supports a great way to make CRUD operations, custom finder methods without need of boilerplate code.

Custom query with @Query annotation:
Spring JPA @Query example: Custom query in Spring Boot

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:

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

You can also know:

Validate Request Body in Spring Boot
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.

Happy learning! See you again.

Source Code

You can find the complete source code for this tutorial on Github.

Using @OneToMany instead: JPA One To Many Unidirectional example
Many-to-Many: JPA Many to Many example with Hibernate in Spring Boot
One-to-One: JPA One To One example with Hibernate in Spring Boot

You can apply this implementation in following tutorials: