Question

我目前正在开发一个java ee应用程序，但我遇到了JPA的问题。我有两个实体：

@Entitiy
public class Restaurant implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    private List<Desk> desks;
}

@Entitiy
public class Desk implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @ManyToOne
    @JoinColumn(name = "restaurant_id")
    private Restaurant restaurant;
}

我正在使用以下代码存储办公桌：

Desk desk = new Desk();
desk.setNumber(Integer.toString(x));
desk.setRestaurant(restaurant);
em.persist(desk);

但现在奇怪的是，实体餐厅的办公桌清单是空的，但实体服务台的餐厅价值是正确的。数据库shema看起来像这样：

RESTAURANT (ID)
DESK (ID, RESTAURANT_ID)
RESTAURANT_DESK (RESTAURANT_ID, DESK_ID)

表RESTAURANT_DESK始终为空。为什么生成第三个表？为什么实体餐厅的办公桌清单是空的？

Answer 1

对于双向映射，您需要在mappedBy实体中提供Restaurant属性以指示反向关系：

@Entity
public class Restaurant implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, mappedBy="restaurant")
    private List<Desk> desks;
}

Answer 2

我发现了问题。我不得不将实体台也添加到餐馆列表中。

Desk desk = new Desk();
desk.setNumber(Integer.toString(x));
desk.setRestaurant(restaurant);
restaurant.getDesks().add(desk);
em.persist(desk);

没有连接表的JPA OneToMany双向

2 个答案: