Long time ago, I wrote a post @OneToOne with shared primary key. Today I would like to return to this problem, with solution based on @MapsId annotation introduced in JPA 2.0

Again we have two entities: Primus and Secundus. Both entities have primary key using Long Java type. They are related 1-1, and Secundus should use the same primary key as Primus.
3 Years after my initial post they will look slightly different ;)

@Entity
@Table(name = "PRIMUS")
public class Primus {

    public static Primus newInstance() {
        Primus primus = new Primus();
        primus.secundus = new Secundus(primus);
        return primus;
    }

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

    @OneToOne(cascade = CascadeType.ALL, mappedBy = "primus")
    private Secundus secundus;

    Primus() {
        super();
    }
    ...

}

Not much changed here, ;). Really important changes were made on the Secundus:

@Entity
@Table(name = "SECUNDUS")
public class Secundus {

    @Id
    private Long id;

@JoinColumn(name = "ID")
    @OneToOne
    @MapsId
    private Primus primus;

    Secundus() {
        super();
    }

    public Secundus(Primus primus) {
        this();
        this.primus = primus;
    }

As you see, the @PrimaryKeyJoinColumn annotation is replaced with two annotations: @MapsId, which defines thatSecundus identifier will be determined by Primus identifier, and @JoinColumn, specifying which column in SECUNDUS table will be used for joining.

Nothing more is needed :) - JPA Provider should automatically ask Primus for its identifier, when persistingSecundus, as long as you take care of correct entities correlation, like in newInstance method of Primus.

Long live JPA 2.0+ ;) :)