01 October 2020

Some time you may need to keep reference by class and reference by ID for your JPA @Entity. It can be very helpful, for example, to do some default JSON serialization with no risk to stuck with well known N+1 issue. In this case i would like to avoid @OneToMany and @ManyToOne fields serialization by default and use ID reference instead.

So, below is simple example how to do above.

@Table(name = "book")
public class Book {

    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

    @Column(name = "name")
    private String name;

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

    @Column(name = "author")
    private long authorId;
    public void setAuthor(Author author) {
        this.author = author;

Testing time:

public class ManyToOneTest {
    public void relationManyToOneTest() {
        EntityManager em = Persistence.createEntityManagerFactory("myDSTest").createEntityManager();

        List <Book> books =em.createQuery("FROM Book", Book.class).getResultList();
        // lazy loading test
        for (Book b : books) {
            System.out.println("Bookd:" + b.getName());
            System.out.println("AuthorId:" + b.getAuthorId());
            // lazy load
            System.out.println("Author:" + b.getAuthor());
        // JPQL with direct id reference
        books = em.createQuery("FROM Book where authorId = 1", Book.class).getResultList();
        // JPQL with author.id reference
        books = em.createQuery("FROM Book where author.id = 1", Book.class).getResultList();

    private void generateTestData(EntityManager em) {

        Author author = new Author();
        author.setName("A Name");

        Book book = new Book();
        book.setName("Book Name");

Test Output:

Hibernate: select book0_.id as id1_2_, book0_.author as author2_2_, book0_.name as name3_2_ from book book0_
Bookd:Book Name
# call book.getAuthor():
Hibernate: select author0_.id as id1_0_0_, author0_.name as name2_0_0_ from author author0_ where author0_.id=?
Author:Author{id=1, name=A Name}

PS: book.getAuthor().getId() will not trigger Author lazy loading, but book.getAuthor().getName() will.

Source code of described application available on GitHub