如何获得自动生成的值？

Question

@TableGenerator(name = "trial", table = "third",
        pkColumnName = "a" , valueColumnName = "b", pkColumnValue = "first")

@Entity

public class First{

    @Id
    @GeneratedValue(strategy = GenerationType.TABLE, generator = "trial")
    protected int a;

    @OneToMany(mappedBy ="first", cascade = CascadeType.PERSIST)
    @JoinColumn(name = "a")
    protected List<Second> seconds;
}

@Entity

public class Second{

    @Id
    protected int a;

    @ManyToOne
    @JoinColumn(name = "b")
    First first;
}

当我在 main 中运行以下命令时：

Second aSecond = new Second();

aSecond.a = 2;

First aFirst = new First();

List<Second> scnds = new ArrayList<Second>();
scnds.add(aSecond);

aFirst.seconds = scnds;

aEntityManager.getTransaction().begin();

aEntityManager.persist(aFirst);

aEntityManager.getTransaction().commit();

它在表第二的“b”列中设置了null？ 如何获取在First 中的a 中自动创建的值以在b 中的Second 中设置？

Answer 1

有几点：

First：您不需要在 First 类中的 seconds 集合上添加 @JoinColumn(name = "a") 注释。映射由 Second 类处理。

下一步：

Second aSecond = new Second();
aSecond.a = 2;
First aFirst = new First();

//its a bi-directional mapping so you have to set the references both ways
aFirst.seconds.add(aSecond);
aSecond.first = aFirst;

aEntityManager.getTransaction().begin();
aEntityManager.persist(aFirst);
aEntityManager.getTransaction().commit();