Composite Primary Keys in JPA

1. Introduction

In this tutorial, we’ll learn about Composite Primary Keys and the corresponding annotations in JPA.

2. Composite Primary Keys

A composite primary key – also called a composite key – is a combination of two or more columns to form a primary key for a table.

In JPA, we have two options to define the composite keys: The @IdClass and @EmbeddedId annotations.

In order to define the composite primary keys, we should follow some rules:

• The composite primary key class must be public
• It must have a no-arg constructor
• It must define equals() and hashCode() methods
• It must be Serializable

3. The IdClass Annotation

Let’s say we have a table called Account and it has two columns – accountNumber, accountType – that form the composite key. Now we have to map it in JPA.

As per the JPA specification, let’s create an AccountId class with these primary key fields:

public class AccountId implements Serializable {
    private String accountNumber;

    private String accountType;

    // default constructor

    public AccountId(String accountNumber, String accountType) {
        this.accountNumber = accountNumber;
        this.accountType = accountType;
    }

    // equals() and hashCode()
}

Next, let’s associate the AccountId class with the entity Account.

In order to do that, we need to annotate the entity with the @IdClass annotation. We must also declare the fields from the AccountId class in the entity Account and annotate them with @Id:

@Entity
@IdClass(AccountId.class)
public class Account {
    @Id
    private String accountNumber;

    @Id
    private String accountType;

    // other fields, getters and setters
}

4. The EmbeddedId Annotation

@EmbeddedId is an alternative to the @IdClass annotation.

Let’s consider another example where we have to persist some information of a Book with title and language as the primary key fields.

In this case, the primary key class, BookId, must be annotated with @Embeddable:

@Embeddable
public class BookId implements Serializable {
    private String title;
    private String language;

    // default constructor

    public BookId(String title, String language) {
        this.title = title;
        this.language = language;
    }

    // getters, equals() and hashCode() methods
}

Then, we need to embed this class in the Book entity using @EmbeddedId:

@Entity
public class Book {
    @EmbeddedId
    private BookId bookId;

    // constructors, other fields, getters and setters
}

5. @IdClass vs @EmbeddedId

As we just saw, the difference on the surface between these two is that with @IdClass, we had to specify the columns twice – once in AccountId and again in Account. But, with @EmbeddedId we didn’t.

There are some other tradeoffs, though.

For example, these different structures affect the JPQL queries that we write.

For example, with @IdClass, the query is a bit simpler:

SELECT account.accountNumber FROM Account account

With @EmbeddedId, we have to do one extra traversal:

SELECT book.bookId.title FROM Book book

Also, @IdClass can be quite useful in places where we are using a composite key class that we can’t modify.

Finally, if we’re going to access parts of the composite key individually, we can make use of @IdClass, but in places where we frequently use the complete identifier as an object, @EmbeddedId is preferred.

6. Conclusion

In this quick article, we explore composite primary keys in JPA.