java 没有外键但有反向外键的休眠多对一关系

Hibernate Many-To-One Relationship without Foreign Key but with reverse foreign key

提问by GingerHead

I have the following DB:

我有以下数据库：

CREATE TABLE car_owner (
  car_owner_id int(11) NOT NULL,
  car_id_fk int(11) DEFAULT NULL,
  PRIMARY KEY (car_owner_id),
  KEY car_owner_car_fk_idx (car_id_fk),
  CONSTRAINT car_owner_car_fk FOREIGN KEY (car_id_fk) REFERENCES car (car_id) ON DELETE NO ACTION ON UPDATE NO ACTION,
) ENGINE=InnoDB DEFAULT CHARSET=latin1;


CREATE TABLE car (
   car_id int(11) NOT NULL AUTO_INCREMENT,
   car_type varchar(45) DEFAULT NULL,
   car_plates varchar(25) DEFAULT NULL,
  PRIMARY KEY (car_id),
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=latin1;

And In the java model:

而在java模型中：

For CarOwner I have:

对于 CarOwner，我有：

@Entity
@Table(name="car_owner")
@NamedQuery(name="CarOwner.findAll", query="SELECT co FROM CarOwner co")
public class CarOwner implements Serializable {

    @Id
    @GeneratedValue
    @Column(name="car_owner_id")
    private Integer carOwnerId;
.....

    //bi-directional many-to-one association to Car
    @OneToMany(fetch = FetchType.EAGER)
    @JoinColumn(name = "car_id_fk", referencedColumnName = "car_id")
    private List<Car> cars;

And for Car:

对于汽车：

@Entity
@Table(name="car")
@NamedQuery(name="Car.findAll", query="SELECT c FROM Car c")
public class Car implements Serializable {

    @Id
    @GeneratedValue
    @Column(name="car_id")
    private Integer carId;
......

    //bi-directional many-to-one association to car_owner
    @ManyToOne(fetch=FetchType.EAGER)
    @JoinColumn(name = "car_owner_id")
    private CarOwner carOwner;

The problem here is that Hibernate can't relate the tables and goes creating new car_idand car_owner_idcolumns in car table automatically.

这里的问题是 Hibernate 无法关联表并自动在 car 表中创建新列car_id和car_owner_id列。

Can anybody help in finding the right combination in the model to relate the tables appropriately.

任何人都可以帮助在模型中找到正确的组合以适当地关联表格。

采纳答案by Shahzeb

@JoinColumnshould be in owner of relationship (in a one to many it's the many side that's regarded the owner).

@JoinColumn应该在关系的所有者中（在一对多中，多方被视为所有者）。

So I will modify this in car

所以我会修改这个 car

 @ManyToOne(fetch=FetchType.EAGER)
 @JoinColumn(name = "car_owner_id",insertable=false, updatable=false)
 private CarOwner carOwner;

And this in CarOwner

而这在 CarOwner

@OneToMany(fetch = FetchType.EAGER,mappedBy = "carOwner")
 private List<Car> cars;

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As a side note I would also not be using EAGERbut that has got nothing to do with question.

作为旁注，我也不会使用，EAGER但这与问题无关。

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Both tables knowing about each other is called Bi-directional relationship.This happens when each table has a key to other table. This is what your java code is expecting.Your tables in database however have a uni-directional relationship. Meaning one table knows about the other but not both. Your car_ownerknows about carbecause of foriegn key CONSTRAINT car_owner_car_fk FOREIGN KEYbut your cardoes not have any idea about car_ownerboth are perfectly valid.

两个表相互了解称为双向关系。当每个表都有另一个表的键时会发生这种情况。这是您的 java 代码所期望的。然而，您在数据库中的表具有单向关系。意思是一张桌子知道另一张桌子，但不知道两者。你car_owner知道car因为外键，CONSTRAINT car_owner_car_fk FOREIGN KEY但你car不知道car_owner两者都是完全有效的。

Now the problem is that in your Java code you are treating it as a bi-directional relationship.

现在的问题是，在您的 Java 代码中，您将其视为双向关系。

//bi-directional many-to-one association to car_owner
 @ManyToOne(fetch=FetchType.EAGER)
 @JoinColumn(name = "car_owner_id")
 private CarOwner carOwner;

but car does not have car_owner_idwhy are you treating it a Bi-directional relationship.

但汽车没有car_owner_id你为什么把它当作双向关系。

Now either update database to make them bi or change java code.

现在要么更新数据库使它们成为 bi 要么更改 java 代码。