Imagine having a tool that can automatically detect if you are using JPA and Hibernate properly. Hypersistence Optimizer is that tool!
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.
For the upcoming test cases, we are going to use the following entities:
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.
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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