How to map the latest child of a parent entity using Hibernate @JoinFormula

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Introduction

Today, I stumbled upon a StackOverflow answer that I gave some time ago and realized that it deserves a post of its own.

As previously explained, the @JoinFormula is a very awesome annotation which allows you to customize the way you join entities beyond JPA @JoinColumn capabilities.

Domain Model

For the upcoming test cases, we are going to use the following entities:

Post and latest PostComment

The PostComment entity is mapped as follows:

@Entity(name = "PostComment")
@Table(name = "post_comment")
public class PostComment {

    @Id
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    private Post post;

    private String review;

    @Column(name = "created_on")
    @Temporal(TemporalType.TIMESTAMP)
    private Date createdOn;

    //Getters and setters omitted for brevity
}

Not only the PostComment has a @ManyToOne association to a Post, but the Post is also associated with the latest PostComment as follows:

@Entity(name = "Post")
@Table(name = "post")
public class Post {

    @Id
    private Long id;

    private String title;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinFormula("(" +
        "SELECT pc.id " +
        "FROM post_comment pc " +
        "WHERE pc.post_id = id " +
        "ORDER BY pc.created_on DESC " +
        "LIMIT 1" +
    ")")
    private PostComment latestComment;

    //Getters and setters omitted for brevity
}

The latestComment attribute associates the parent Post entity with the latest PostComment child entity. The @JoinFormula annotation allows us to define any SQL select query to provide the relationship between two entities.

Testing time

Considering we have the following entities in our database:

Post post = new Post();
post.setId(1L);
post.setTitle("High-Performance Java Persistence");
entityManager.persist(post);
assertNull(post.getLatestComment());

PostComment comment1 = new PostComment();
comment1.setId(1L);
comment1.setPost(post);
comment1.setCreatedOn(Timestamp.valueOf(
    LocalDateTime.of(2016, 11, 2, 12, 33, 14)
));
comment1.setReview("Woohoo!");
entityManager.persist(comment1);

PostComment comment2 = new PostComment();
comment2.setId(2L);
comment2.setPost(post);
comment2.setCreatedOn(Timestamp.valueOf(
    LocalDateTime.of(2016, 11, 2, 15, 45, 58)
));
comment2.setReview("Finally!");
entityManager.persist(comment2);

PostComment comment3 = new PostComment();
comment3.setId(3L);
comment3.setPost(post);
comment3.setCreatedOn(Timestamp.valueOf(
    LocalDateTime.of(2017, 2, 16, 16, 10, 21)
));
comment3.setReview("Awesome!");
entityManager.persist(comment3);

When we fetch the Post entity, we can see that the latestComment attribute works as expected:

Post post = entityManager.find(Post.class, 1L);
PostComment latestComment = post.getLatestComment();

assertEquals("Awesome!", latestComment.getReview());

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Conclusion

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10 Comments on “How to map the latest child of a parent entity using Hibernate @JoinFormula

  1. Hi Vlad, unfortunately any parent entity won’t be returned if it has no ‘latest’ child record (or any record matching the condition in the JoinFormula). I’ve just tested that in your test: I added another Post without any PostComments and at the bottom of the test this post is not returned.
    What is the best way to return it and have the ‘latest’ record simply null? Thanks.

      • If you explicitly ask for Post with id 2, then yes. If you ask for all posts, you only get the one that has the ‘latest’ record. I’ve created a PR with a simple test to demonstrate it.

      • That’s the expected behaviour when you use JOIN which means INNER JOIN. Try with LEFT JOIN if you want to get all Posts, no matter whether they have comments.

  2. There is a @ManyToOne association in the Post entity. I wonder why @JoinFormula can’t use with @OneToOne?

    • There’s a bug in Hibernate annotation processing preventing you from using a @OneToOne association with @JoinFormula.

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