jpa-composite-primary-keys

Posted on: 2019-10-08 2019-11-19
Tags: JPA

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.

As always, the complete code for this article can be found over on Github.

getdocs

3043