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Spring Boot One-to-Many and Many-to-One Mapping
- A One-to-Many relationship means one entity is related to multiple entities.
- A Many-to-One relationship means multiple child entities are related to a single parent entity.
Example:
✅ A User can have multiple Posts
✅ A Post belongs to only one User
Define User and Post Entities
We'll create a bidirectional One-to-Many relationship between User and Post.
User.java
import com.fasterxml.jackson.annotation.JsonManagedReference;
import jakarta.persistence.*;
import java.util.List;
@Entity
public class User {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
@OneToMany(mappedBy = "user", cascade = CascadeType.ALL, orphanRemoval = true)
@JsonManagedReference // 🔥 Prevents infinite recursion
private List posts;
// Getters and Setters
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List getPosts() {
return posts;
}
public void setPosts(List posts) {
this.posts = posts;
}
}
✅ @OneToMany(mappedBy = "user") → Specifies that User is the parent.
✅ cascade = CascadeType.ALL → Ensures changes in User reflect in Post.
✅ orphanRemoval = true → Automatically deletes orphaned Post entries when removed from User.
✅ @JsonManagedReference → This marks the owning side (parent).
Post.java
import com.fasterxml.jackson.annotation.JsonBackReference;
import jakarta.persistence.*;
@Entity
public class Post {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String title;
@ManyToOne
@JoinColumn(name = "user_id", nullable = false)
@JsonBackReference // 🔥 Prevents infinite recursion
private User user;
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
}
✅ @ManyToOne → Defines that Post belongs to a User.
✅ @JoinColumn(name = "user_id") → Creates a foreign key user_id in the Post table.
✅ @JsonBackReference → This marks the child side, preventing it from serializing the User field.
Table name : user
| id | name |
|---|
Table name : post
| id | title | user_id |
|---|
Create JPA Repositories
UserRepository.java
import org.springframework.data.jpa.repository.JpaRepository;
public interface UserRepository extends JpaRepository {
}
PostRepository.java
import org.springframework.data.jpa.repository.JpaRepository;
public interface PostRepository extends JpaRepository {
}
Create Services
UserService.java
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import java.util.List;
import java.util.Optional;
@Service
public class UserService {
@Autowired
private UserRepository userRepository;
@Autowired
private PostRepository postRepository;
// Create User with Posts
public User createUserWithPosts(User user) {
if (user.getPosts() != null) {
for (Post post : user.getPosts()) {
post.setUser(user); // 🔥 Automatically assign user to posts
}
};
return userRepository.save(user);
}
// Get Users
public List getUsersWithPosts() {
return userRepository.findAll();
}
// Get User by ID
public Optional getUserById(Long id) {
return userRepository.findById(id);
}
// Update User
public User updateUser(User user) {
return userRepository.save(user);
}
// Delete User (Cascade deletes Posts)
public void deleteUser(Long id) {
userRepository.deleteById(id);
}
}
PostService.java
import org.springframework.stereotype.Service;
import java.util.List;
import java.util.Optional;
@Service
public class PostService {
private final PostRepository postRepository;
private final UserRepository userRepository;
public PostService(PostRepository postRepository, UserRepository userRepository) {
this.postRepository = postRepository;
this.userRepository = userRepository;
}
// Create Post for a specific User
public Post createPost(Long userId, Post post) {
Optional user = userRepository.findById(userId);
if (user.isPresent()) {
post.setUser(user.get());
return postRepository.save(post);
} else {
throw new RuntimeException("User not found!");
}
}
// Get Posts by User ID
public List getPostsByUser(Long userId) {
return postRepository.findAll().stream()
.filter(post -> post.getUser().getId().equals(userId))
.toList();
}
// Delete Post by ID
public void deletePost(Long postId) {
postRepository.deleteById(postId);
}
}
Create Controllers
UserController.java
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;
import java.util.List;
import java.util.Optional;
@RestController
@RequestMapping("/users")
public class UserController {
@Autowired
private UserService userService;
// Create User with Posts
@PostMapping
public User createUser(@RequestBody User user) {
return userService.createUserWithPosts(user);
}
// Create User with Posts
@GetMapping
public List getUsers() {
return userService.getUsersWithPosts();
}
// Get User by ID
@GetMapping("/{id}")
public Optional getUser(@PathVariable Long id) {
return userService.getUserById(id);
}
// Get User by ID
@PutMapping
public User updateUser(@RequestBody User user) {
return userService.updateUser(user);
}
// Delete User (Cascade deletes Posts)
@DeleteMapping("/{id}")
public String deleteUser(@PathVariable Long id) {
userService.deleteUser(id);
return "Deleted Successfully";
}
}
PostController.java
import org.springframework.web.bind.annotation.*;
import java.util.List;
@RestController
@RequestMapping("/posts")
public class PostController {
private final PostService postService;
public PostController(PostService postService) {
this.postService = postService;
}
// Create Post for a User
@PostMapping("/{userId}")
public Post createPost(@PathVariable Long userId, @RequestBody Post post) {
return postService.createPost(userId, post);
}
// Get Posts by User ID
@GetMapping("/user/{userId}")
public List getPostsByUser(@PathVariable Long userId) {
return postService.getPostsByUser(userId);
}
// Delete Post by ID
@DeleteMapping("/{postId}")
public void deletePost(@PathVariable Long postId) {
postService.deletePost(postId);
}
}
Testing the REST API with Postman for one-to-many:
Once the application is running, test the endpoints:
Create User http://localhost:8080/users
Get All Users http://localhost:8080/users
Get User By Id http://localhost:8080/users/{userID}
Update User http://localhost:8080/users
Delete User http://localhost:8080/users/{userID}
Testing the REST API with Postman for many-to-one:
Once the application is running, test the endpoints:
Create Post http://localhost:8080/posts/{userID}
Get Posts by user id http://localhost:8080/posts/{userID}
Delete Posts http://localhost:8080/posts/{postID}
