Simple note about using JPA relation mappings

14 February 2020

There is a lot of typical examples how to build JPA @OneToMany and @ManyToOne relationships in your Jakarta EE application. And usually it looks like:

@Entity
@Table(name = "author")
public class Author {
    @OneToMany
    private List<Book> book;
    ...
}

@Entity
@Table(name = "book")
public class Book {
    @ManyToOne
    private Author author;
    ...
}

This code looks pretty clear, but on my opinion you should NOT USE this style in your real world application. From years of JPA using experience i definitely can say that sooner or later your project will stuck with known performance issues and holy war questions about: N+1, LazyInitializationException, Unidirectional @OneToMany , CascadeTypes ,LAZY vs EAGER, JOIN FETCH, Entity Graph, Fetching lot of unneeded data, Extra queries (for example: select Author by id before persist Book) etcetera. Even if you are have answers for each potential issue above, usually proposed solution will add unreasonable complexity to the project.

To avoid potential issues i recommend to follow next rules:

Avoid using of @OneToMany at all
Use @ManyToOne to build constrains but work with ID instead of Entity

Unfortunately, simple snippet below does not work as expected in case persist

@ManyToOne(targetEntity = Author.class)
private long authorId;

But,we can use next one instead of

@JoinColumn(name = "authorId", insertable = false, updatable = false)
@ManyToOne(targetEntity = Author.class)
private Author author;

private long authorId;

public long getAuthorId() {
    return authorId;
}

public void setAuthorId(long authorId) {
    this.authorId = authorId;
}

Hope, this two simple rules helps you enjoy all power of JPA with KISS and decreasing count of complexity.