@Entity(name = "users")
public class User {
    @Id
    @Column(name = "id")
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
    @MapsId("uuid")
    @JoinColumn(name = "login")
    @ManyToOne(cascade = CascadeType.MERGE, fetch = FetchType.EAGER)
    private Login login;
}

@Entity(name = "login")
public class Login {
    @Id
    @Column(name = "uuid")
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
}