@OneToMany 的错误使用返回空列表 [重复]

Question

我有两个模型类：

拥有车主字段的汽车：

import lombok.Data;

import javax.persistence.*;

@Entity
@Data
public class Car {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String carBrand;
    private String modelOfCar;
    private String gearbox;
    private int yearProduction;
    private int mileage;
    private String status;
    @ManyToOne
    @JoinColumn(name = "owner_id")
    private Owner owner;
}

还有拥有汽车领域的所有者

@Entity
@Data
public class Owner implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String nameOfOwner;
    private String surnameOfOwner;
    @OneToMany(mappedBy = "owner")
   
    private List<Car> cars = new ArrayList<>()
            
    public void addCarToOwner(Car car) {
        cars.add(car);
    }
}

我尝试将一些示例数据添加到该数据中的嵌入式 h2 DB：

    Car car1 = new Car();
    car1.setCarBrand("asd");
    car1.setModelOfCar("asd");
    car1.setGearbox("asd");
    car1.setYearProduction(22018);
    car1.setMileage(321000);
    car1.setStatus("Available");

    Car car2 = new Car();
    car2.setCarBrand("asd");
    car2.setModelOfCar("asd");
    car2.setGearbox("asd");
    car2.setYearProduction(2011);
    car2.setMileage(6801020);
    car2.setStatus("Available");

  Owner owner1 = new Owner();
    owner1.setNameOfOwner("asd");
    owner1.setSurnameOfOwner("asd");
    owner1.addCarToOwner(car1);

    Owner owner2 = new Owner();
    owner2.setNameOfOwner("asd");
    owner2.setSurnameOfOwner("asd");
    owner2.addCarToOwner(car2);

  carRepository.save(car1);
    carRepository.save(car2);
    ownersRepository.save(owner1);
    ownersRepository.save(owner2);

当我使用findAll() 方法检查所有者存储库的状态时，我得到所有者和他的汽车的空列表。当我为汽车存储库运行findAll() 时，我的所有者为空，因为我没有设置它，但是当所有者需要汽车并且汽车需要所有者时如何设置它？

owner repository

car repository

所有这些导致当我尝试获取具有给定 ID 的用户时，我得到一个空的汽车列表。

Answer 1

看起来您正在Owner 和Car 之间创建一个双向OneToMany。在这种情况下，您需要同步关联的双方。

您的实用方法addCarToOwner 可以很好地做到这一点：

@Data
@Entity
public class Owner implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String nameOfOwner;
    private String surnameOfOwner;
    @OneToMany(mappedBy = "owner")
    private List<Car> cars = new ArrayList<>()
    
    public void addCarToOwner(Car car) {
        car.setOwner(this);
        this.cars.add(car);
    }
}

通过将Owner 设置为Car，您可以同步关联的双方。