JPA One To Many example with Hibernate and Spring Boot

In this tutorial, I will show you how to implement Spring JPA One-To-Many mapping with Hibernate in a Spring Boot CRUD 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 One-To-Many relationship
  • Way to use Spring JPA to interact with Database for One-To-Many association
  • Way to create Spring Rest Controller to process HTTP requests

Related Posts:
JPA Many to Many example with Hibernate in Spring Boot
Spring Boot Token based Authentication with Spring Security & JWT
Spring JPA + H2 example
Spring JPA + MySQL example
Spring JPA + PostgreSQL example
Spring JPA + Oracle example
Spring JPA + SQL Server example

Appropriate way to implement JPA/Hibernate 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.

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 One To Many 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:

jpa-one-to-many-diagram

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

These are APIs that we need to provide:

MethodsUrlsActions
POST/api/tutorials/:id/commentscreate new Comment for a Tutorial
GET/api/tutorials/:id/commentsretrieve all Comments of a Tutorial
GET/api/comments/:idretrieve a Comment by :id
PUT/api/comments/:idupdate a Comment by :id
DELETE/api/comments/:iddelete a Comment by :id
DELETE/api/tutorials/:iddelete a Tutorial (and its Comments) by :id
DELETE/api/tutorials/:id/commentsdelete all Comments of a Tutorial

Assume that we’ve had tutorials table like this:

jpa-one-to-many-parent-table

Here are the example requests:

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

jpa-one-to-many-example-create-child-entity

comments table after that:

jpa-one-to-many-child-table

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

jpa-one-to-many-example-spring-crud-retrieve

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

jpa-one-to-many-example-spring-crud-delete

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

jpa-one-to-many-example-spring-crud-table-delete

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

jpa-one-to-many-example-spring-crud-delete-cascade

jpa-one-to-many-example-spring-crud-delete-parent-table

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

jpa-one-to-many-example-spring-crud-delete-cascade-table

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

Spring Boot One to Many example

Technology

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

Project Structure

jpa-one-to-many-example-hibernate-spring-boot-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>mysql</groupId>
	<artifactId>mysql-connector-java</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.MySQL5InnoDBDialect
# 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 MySQL5InnoDBDialect 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 One to Many 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 javax.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 javax.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).

By default, the @ManyToOne association uses FetchType.LAZY for fetch type:

jpa-one-to-many-lazy-fetch

You can set it to EAGER 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;
  ...
}

jpa-one-to-many-eager-fetch

Create Repository Interfaces for One To Many 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 javax.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:

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 Repositiory 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 One to Many 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:
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.

Happy learning! See you again.

Further Reading

Fullstack CRUD App:
Vue + Spring Boot example
Angular 8 + Spring Boot example
Angular 10 + Spring Boot example
Angular 11 + Spring Boot example
Angular 12 + Spring Boot example
Angular 13 + Spring Boot example
React + Spring Boot example

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

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

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

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