spring-data-jpa-skill

name	spring-data-jpa-skill
description	Implement the persistence layer using Spring Data JPA in Spring Boot applications.

Follow the below principles when using Spring Data JPA:

1. Disable the Open Session in View (OSIV) filter: spring.jpa.open-in-view=false

2. Disable in-memory pagination: spring.jpa.properties.hibernate.query.fail_on_pagination_over_collection_fetch=true

3. Avoid the N+1 SELECT problem: use JOIN FETCH to load associated child collections in a single query.

4. Avoid in-memory pagination: when loading a paginated list of parent entities with child collections:

   • First, load only the parent IDs using pagination
   • Then, load the full entities with their child collections using JOIN FETCH for those IDs
   • Assemble the final Page from the paginated IDs and the loaded entities

Pagination with child collections example:

PostRepository.java

public interface PostRepository extends JpaRepository<Post, Long> {

@Query("select p.id from Post p order by p.id") Page findPostIds(Pageable pageable);

@Query("select distinct p from Post p left join fetch p.comments where p.id in :ids") List findAllByIdInWithComments(@Param("ids") Collection ids); }

PostService.java

@Service public class PostService { private final PostRepository postRepository;

public PostService(PostRepository postRepository) { this.postRepository = postRepository; }

@Transactional(readOnly = true) public Page findPosts(Pageable pageable) { Page idsPage = postRepository.findPostIds(pageable); if (idsPage.isEmpty()) { return Page.empty(pageable); } List posts = postRepository.findAllByIdInWithComments(idsPage.getContent()); return new PageImpl<>(posts, pageable, idsPage.getTotalElements()); } }