Jpa

The “object references an unsaved transient instance” error is common in Hibernate. Here’s how to fix it. Understanding the Error This error occurs when you try to save an entity that references another entity that hasn’t been persisted yet. // Error scenario User user = new User(); // Transient (not saved) Post post = new Post(); post.setUser(user); // References unsaved user postRepository.save(post); // Error! Solutions 1. Save Parent First // Save user first User user = new User(); user = userRepository.save(user); // Now managed Post post = new Post(); post.setUser(user); postRepository.save(post); // Works! 2. Use Cascade Types @Entity public class Post { @ManyToOne(cascade = CascadeType.PERSIST) private User user; } // Now saving post will save user too Post post = new Post(); post.setUser(new User()); postRepository.save(post); // Works! 3. Use @OneToMany with Cascade @Entity public class User { @OneToMany(mappedBy = "user", cascade = CascadeType.ALL) private List<Post> posts; } User user = new User(); Post post = new Post(); user.getPosts().add(post); post.setUser(user); userRepository.save(user); // Saves both Common Patterns Bidirectional Relationships @Entity public class User { @OneToMany(mappedBy = "user", cascade = CascadeType.ALL) private List<Post> posts = new ArrayList<>(); } @Entity public class Post { @ManyToOne @JoinColumn(name = "user_id") private User user; // Helper method public void setUser(User user) { this.user = user; if (user != null && !user.getPosts().contains(this)) { user.getPosts().add(this); } } } Best Practices Save parent entities first Use appropriate cascade types Maintain bidirectional relationships Use helper methods for relationships Handle null references Conclusion Fix Hibernate transient instance errors by: ...

The N+1 problem is a common performance issue in JPA. Here’s how to solve it in Spring Data JPA. Understanding N+1 Problem // N+1 queries: 1 for users + N for each user's posts List<User> users = userRepository.findAll(); for (User user : users) { List<Post> posts = postRepository.findByUserId(user.getId()); // N queries } Solutions 1. Eager Fetching @Entity public class User { @OneToMany(fetch = FetchType.EAGER) private List<Post> posts; } 2. Join Fetch (JPQL) @Query("SELECT u FROM User u JOIN FETCH u.posts") List<User> findAllWithPosts(); 3. Entity Graphs @EntityGraph(attributePaths = {"posts"}) @Query("SELECT u FROM User u") List<User> findAllWithPosts(); 4. Batch Fetching @Entity public class User { @BatchSize(size = 10) @OneToMany(fetch = FetchType.LAZY) private List<Post> posts; } Best Practices Use lazy loading by default Fetch associations when needed Use entity graphs Monitor query performance Use DTO projections Conclusion Solve N+1 problems with: ...