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
Related Posts:
- JPA One To One example with Hibernate in Spring Boot
- JPA Many to Many example with Hibernate in Spring Boot
- Validate Request Body 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
- Documentation: Spring Boot Swagger 3 example
- Caching: Spring Boot Redis Cache example
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 Comment
s) 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 orPostgreSQLDialect
for PostgreSQL -
spring.jpa.hibernate.ddl-auto
is used for database initialization. We set the value toupdate
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 bevalidate
.
- 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 andjdbc: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 urlhttp://localhost:8080/h2-console
will change tohttp://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 keygenerator
. 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 withpublished
having value as inputpublished
. -
findByTitleContaining()
: returns all Tutorials which title contains inputtitle
. -
findByTutorialId()
: returns all Comments of a Tutorial specified bytutorialId
. -
deleteByTutorialId()
: deletes all Comments of a Tutorial specified bytutorialId
.
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 forCommentRepository
.
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:
- 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 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.
Further Reading
- Secure Spring Boot App with Spring Security & JWT Authentication
- Spring Data JPA Reference Documentation
- Spring Boot Pagination and Sorting example
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
- Angular 14 + Spring Boot example
- Angular 15 + Spring Boot example
- Angular 16 + 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
One-to-One: JPA One To One 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
Documentation: Spring Boot + Swagger 3 example (with OpenAPI 3)
Caching: Spring Boot Redis Cache example
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