java - Hibernate - @ManyToOne 注释创建附加列

Question

所以我正在学习有关 hibernate 的教程，因为我想在即将进行的项目中使用该框架。我在使用 @ManyToOne 和 @OneToMay 注释以及双向映射时遇到问题。我想做的是非常基本的东西。我只想要两张表，一张代表用户，另一张代表车辆。现在我希望用户能够拥有许多车辆，并且汽车应该“知道”它所属的用户。

因为创建一个额外的表来保存用户和车辆之间的连接似乎是“最佳实践”。我尝试使用 @JoinTable 批注(见下文)来执行此操作，但它无法正常工作。

为此，我编写了以下类:

UserDetails.java

@Entity
@Table(name="USER_DETAILS")
public class UserDetails {
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private int userId;
private String userName;
private String email;
@OneToMany()
@JoinTable(name="USER_VEHICLE", joinColumns=@JoinColumn(name="USER_ID"),
            inverseJoinColumns=@JoinColumn(name="VEHICLE_ID")
)
private Collection<Vehicle> vehicle = new ArrayList<Vehicle>();

...
getters and setters

车辆.java

@Entity
public class Vehicle {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int vehicleId;
private String vehicleName;
@ManyToOne  
private UserDetails user;

...
getters and setters

测试上面的类:

HibernateTest.Java

public class HibernateTest {
/**
 * 
 * @param args
 */
public static void main(String[] args){

    UserDetails user1 = new UserDetails();      
    user1.setUserName("testUser1");
    user1.setEmail("testEmail1");


    Vehicle vehicle1 = new Vehicle();
    vehicle1.setVehicleName("Car");

    Vehicle vehicle2 = new Vehicle();
    vehicle2.setVehicleName("Jeep");

    user1.getVehicle().add(vehicle1);
    user1.getVehicle().add(vehicle2);
    vehicle1.setUser(user1);
    vehicle2.setUser(user1);    

    SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
    Session session = sessionFactory.openSession();
    session.beginTransaction();
    session.save(user1);
    session.save(vehicle1);
    session.save(vehicle2);
    session.getTransaction().commit();
    session.close();

当我运行它时，它执行时没有任何错误或异常，并创建了下表:

USER_DETAILS
+------+----------+---------+
|userId|email     |userName |
+------+----------+---------+
|1     |testEmail1|testUser1|
+------+----------+---------+

VEHICLE
+---------+-----------+-----------+
|vehicleId|vehicleName|user_userId| <- did not expect this column
+---------+-----------+-----------+
|1        |Car        |1          |
+---------+-----------+-----------+
|2        |Jeep       |1          |
+---------+-----------+-----------+

USER_VEHICLE
 +------+----------+
|USER_ID|VEHICLE_ID|
+-------+----------+
|1      |1         |
+-------+----------+
|1      |2         |
+-------+----------+

据我了解，由 hibernate 创建的附加列“user_userId”似乎是不必要的，因为用户和车辆之间的连接已经在“USER_VEHICLE”表中表示，我不打算再增加这个专栏已创建。

是不是我做双向映射的方式不对？在不创建附加列的情况下使其工作的正确方法是什么？

Answer 1

如果你想要一个@ManyToMany 关系，

@JoinTable 是必需的。

@Entity
public class UserDetails {
    /* ... */

    @ManyToMany
    @JoinTable(name = "user_vehicle",
        joinColumns = @JoinColumn(name = "user_id"),
        inverseJoinColumns = @JoinColumn(name = "vehicle_id"))
    private List<Vehicle> vehicles;
}

@Entity
public class Vehicle {
    /* ... */
}

USER_DETAILS      VEHICLE           USER_VEHICLE
+---------+       +------------+    +---------+------------+
| user_id |       | vehicle_id |    | user_id | vehicle_id |
+---------+       +------------+    +---------+------------+
|       1 |       |          1 |    |       1 |          1 |
+---------+       +------------+    +---------+------------+

---

因为你不想这样，你可以像这样删除 @JoinTable:

@Entity
public class UserDetails {
    /* ... */

    @OneToMany(mappedBy = "user")
    private List<Vehicle> vehicles;
}

@Entity
public class Vehicle {
    /* ... */

    @ManyToOne
    private UserDetails user;
}

USER_DETAILS      VEHICLE       
+---------+       +------------+---------+
| user_id |       | vehicle_id | user_id |
+---------+       +------------+---------+
|       1 |       |          1 |       1 |
+---------+       +------------+---------+

---

如果您想为 (UserDetails, Vehicle) 元组存储额外数据，您只需要第三个表，在这种情况下，您将创建第三个类。

@Entity
public class UserDetails {
    /* ... */

    @OneToMany(mappedBy = "user")
    private List<UserVehicle> vehicles;
}

@Entity
public class Vehicle {
    /* ... */
}

@Entity
public class UserVehicle {
    /* ... */

    @ManyToOne
    private UserDetails user;

    @ManyToOne
    private Vehicle vehicle;

    @Basic
    private String someString;
}

USER_DETAILS      VEHICLE           USER_VEHICLE
+---------+       +------------+    +---------+------------+-------------+
| user_id |       | vehicle_id |    | user_id | vehicle_id | some_string |
+---------+       +------------+    +---------+------------+-------------+
|       1 |       |          1 |    |       1 |          1 |  Hello      |
+---------+       +------------+    +---------+------------+-------------+