java - JPA - 如何在 EclipseLink 中正确保存 OneToOne 关联？

Question

我正在使用 Tomcat 和 EclipseLink。我想知道，为什么我的关联存储在我的数据库中，但我的实体对象在相应的属性中没有值？我想，我只需要创建两个实体并在事务开始和结束期间调用设置器。我可以在我的数据库中看到，一切顺利！我的预订表中有一个“用户”列，其中包含外键。不幸的是，如果我在预订时调用 toString()-method，那么 booker-property 似乎仍然为空。什么地方出了错？这是我的例子：

我的班级 BookerApi.java

public Booking createBooking(Integer courtNo, Date startDate, Date endDate, Integer bookerId) {
    // Entity bilden
    Booking booking = new Booking();
    booking.setCourtNo(courtNo);
    booking.setStart(startDate);
    booking.setEnd(endDate);
    User booker = new User();
    booker.setEmail("100");
    booker.setName("101");
    booker.setPassword("102");
    emf = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
    EntityManager em = emf.createEntityManager();
    em.getTransaction().begin();
    booking.setBooker(booker); // set relation
    em.persist(booking); // persist relation, entity a, see vogella.com/articles/JavaPersistenceAPI/article.html#example
    em.persist(booker);
    em.getTransaction().commit();
    em.clear();
    emf.close();
    //System.out.println("BookerApi.createBooking() - booking (out of em-context): " + booking);    
    return booking;
}

更新：根据要求，这里是 Booking.java 的代码

@Entity
@Table(name="booking")
public class Booking implements Serializable {
    private static final long serialVersionUID = -5077792582177235184L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    Integer id;
    @Temporal(TemporalType.DATE)
    @Column(nullable = false)
    Date start;
    @Temporal(TemporalType.DATE)
    @Column(nullable = false)
    Date end;
    @Column(nullable = false)
    Integer courtNo;
    @OneToOne(fetch = FetchType.EAGER) // Mueller S. 105, 109
    @JoinColumn(name = "user")
    User booker;
    public Booking(){
    }
    public Integer getId() {
        return id;
    }
    public Date getStart() {
        return start;
    }
    public void setStart(Date start) {
        this.start = start;
    }
    public Date getEnd() {
        return end;
    }
    public void setEnd(Date end) {
        this.end = end;
    }
    public Integer getCourtNo() {
        return courtNo;
    }
    public void setCourtNo(Integer courtNo) {
        this.courtNo = courtNo;
    }
    public User getBooker() {
        return booker;
    }
    public void setBooker(User booker) {
        this.booker = booker;
    }
    public Integer getBookerId() {
        if (getBooker() == null){
            return 0;
        } else {
            return getBooker().getId();
        }
    }
    public void setBookerId(Integer bookerId) {
        //System.out.println("Booking.setBookerId(Integer bookerId) " + bookerId);
        BookerApi api = new BookerApi();
        User booker = api.readUser(bookerId);
        setBooker(booker);
    }
    @Override
    public String toString(){
        String logString = "LogString Booking -" +
            " id: " + getId().toString() + ";" +
            " court: " + getCourtNo().toString() + ";" +
            " start: " + getStart() + ";";
        if (getBooker() == null){
            logString += " booker: " + getBooker() + ";";
        } else {
            logString += " booker: - ;";
        }
        return logString;
    }
}