How to use JPA findBy query on column mapped by @OneToMany relationship?

Question

How to use jpa query to find a record from column which has @OneToMany relationship?

Post.class

public class Post {
  @Id
  private String id;
  ...
  ...
  
  @OneToMany(cascade = CascadeType.ALL, mappedBy = "comment", fetch = FetchType.LAZY)
  private Set<Comment> comments;

}

Comment.class

public class Comment {
  ...
  ...

  @ManyToOne(fetch = FetchType.LAZY, optional = false)
  @JoinColumn(name = "comment", referencedColumnName = "id")
  private Post post;
}

Is there any way to query on PostRepository and find the Post using commentId?

Answer 1

To find by commentId you just have to create a function in your repository interface:

List<Comment> findByPost(Post post);

Then when you query just add:

Post post = new Post();
post.setId(yourId);
repository.findByPost(post);

Answer 2

Assuming that Comment has the property String id you could do this as follows:

public interface PostRepository extends JpaRepository<Post, String> {
    List<Post> findByCommentsId(String id);
}

You can read more about this on:

• https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#repository-query-keywords
• https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#jpa.query-methods.query-creation