Introduction

While answering questions on the Hibernate forum, I stumbled on the following question about using the @ManyToOne annotation when the Foreign Key column on the client side references a non-Primary Key column on the parent side.

In this article, you are going to see how to use the @JoinColumn annotation in order to accommodate non-Primary Key many-to-one associations.

Domain Model

Assuming we have the following tables in our database:

The isbn column on the publication and book tables are linked via a Foreign Key constraint which is the base of our @ManyToOne assocation:

Non Primary-Key @ManyToOne mapping

The Book represents the parent side of the association, and it’s mapped as follows:

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

    @Id
    @GeneratedValue
    private Long id;

    private String title;

    private String author;

    @NaturalId
    private String isbn;

    //Getters and setters omitted for brevity
}

The isbn column is mapped as a @NaturalId since it can be used as a business key as well.

For more details about the @NaturalId annotation, check out this article.

The Publication represents the child of the association, so it’s going to be mapped like this:

@Entity(name = "Publication")
@Table(name = "publication")
public class Publication {

    @Id
    @GeneratedValue
    private Long id;

    private String publisher;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(
        name = "isbn", 
        referencedColumnName = "isbn"
    )
    private Book book;

    @Column(
        name = "price_in_cents", 
        nullable = false
    )
    private Integer priceCents;

    private String currency;

    //Getters and setters omitted for brevity
}

By default, the @ManyToOne association assumes that the parent-side entity identifier is to be used to join with the client-side entity Foreign Key column.

However, when using a non-Primary Key association, the referencedColumnName should be used to instruct Hibernate which column should be used on the parent side to establish the many-to-one database relationship.

Testing time

Assuming we have the following entities in our database:

Book book = new Book();
book.setTitle( "High-Performance Java Persistence" );
book.setAuthor( "Vlad Mihalcea" );
book.setIsbn( "978-9730228236" );
entityManager.persist(book);

Publication amazonUs = new Publication();
amazonUs.setPublisher( "amazon.com" );
amazonUs.setBook( book );
amazonUs.setPriceCents( 4599 );
amazonUs.setCurrency( "$" );
entityManager.persist( amazonUs );

Publication amazonUk = new Publication();
amazonUk.setPublisher( "amazon.co.uk" );
amazonUk.setBook( book );
amazonUk.setPriceCents( 3545 );
amazonUk.setCurrency( "&" );
entityManager.persist( amazonUk );

Upon fetching the Publication along with its associated Book, we can see that the @ManyToOne association works as expected:

Publication publication = entityManager.createQuery(
    "select p " +
    "from Publication p " +
    "join fetch p.book b " +
    "where " +
    "   b.isbn = :isbn and " +
    "   p.currency = :currency", Publication.class)
.setParameter( "isbn", "978-9730228236" )
.setParameter( "currency", "&" )
.getSingleResult();

assertEquals(
    "amazon.co.uk",
    publication.getPublisher()
);

assertEquals(
    "High-Performance Java Persistence",
    publication.getBook().getTitle()
);

When executing the JPQL query above, Hibernate generates the following SQL statement:

SELECT 
        p.id AS id1_1_0_, b.id AS id1_0_1_,
        p.isbn AS isbn5_1_0_, p.currency AS currency2_1_0_,
        p.price_in_cents AS price_in3_1_0_,
        p.publisher AS publishe4_1_0_,
        b.author AS author2_0_1_, b.isbn AS isbn3_0_1_,
        b.title AS title4_0_1_
FROM    publication p
INNER JOIN 
        book b ON p.isbn = b.isbn
WHERE   b.isbn = '978-9730228236'
        AND p.currency = '&'

As you can see, the referencedColumnName allows you to customize the JOIN ON clause so that the isbn column is used instead of the default entity identifier.

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Conclusion

If you want to represent a non-Primary Key @ManyToOne association, you should use the referencedColumnName attribute of the @JoinColumn annotation.

For more complex situations, like when you need to use a custom SQL function in the JOIN ON clause, you can use the Hibernate specific @JoinFormula annotation.

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