合并时Hibernate尝试从null一对一属性中分配id

Question

让我们考虑两个实体：

@Entity
@Table
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    @OneToOne(cascade = CascadeType.ALL, orphanRemoval = true, mappedBy = "user")
    private UserDetails userDetails;

}

@Entity
@Table(name="user_details")
public class UserDetails {
    @GenericGenerator(name = "generator", strategy = "foreign",
        parameters = @Parameter(name = "property", value = "user"))
    @Id
    @GeneratedValue(generator = "generator")
    @Column(unique = true, nullable = false)
    private Integer id;

    @OneToOne
    @PrimaryKeyJoinColumn
    private User user;

    public UserDetails(User user) {
        this.user = user;
        user.setUserDetails(this);
    }
}

并给出代码行：

UserDetails details = new UserDetails(user);
session.merge(details);

产生错误：

org.springframework.orm.jpa.JpaSystemException: attempted to assign id from null one-to-one property [com.app.domain.UserDetails.user]; nested exception is 
org.hibernate.id.IdentifierGenerationException: attempted to assign id from null one-to-one property [com.app.domain.UserDetails.user]

我知道，当我使用方法save（）而不是merge（）时，它会起作用。但是，ethod save（）不能自动更新现有的行，而merge（）可以。