无法使用 Spring Data JPA 创建 JOIN

Question

我是Spring Data的新手，并试图通过关注这篇关于SO和其他一些教程的帖子来解决这个问题，但没有取得太大成功。

我正在尝试在 2 个表之间使用 Spring Data JPA 进行简单的连接。数据库中的表称为： * user_vehicle -

user_vehicle表中数据库中的当前数据：
ID|vehicle_ID|user_ID
1|1|1
2|2|1

这是我尝试过但无法让它工作的代码（getter 和 setter 从帖子中删除以缩短它）：

@Entity(name = "vehicle_model")
public class VehicleModel {

@Id
@Min(1)
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private Long manufacturerId;
private String title;
private int ccm;
private int kw;
private int yearOfManufacture;
private int engineTypeId;
private boolean isActive;

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "vehicle_id", insertable = false, updatable = false)
@Fetch(FetchMode.JOIN)
private UserVehicle userVehicle;
}


@Entity(name = "user_vehicle")
public class UserVehicle {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private long id;
@Column(nullable = false)
private long vehicleId;
@Column(nullable = false)
private long userId;

@OneToMany(targetEntity = VehicleModel.class, mappedBy = "userVehicle", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
List<VehicleModel> vehicleModels;
}


@Repository
public interface UserVehicleRepository extends CrudRepository<UserVehicle, Long> 
{
    Iterable<UserVehicle> findVehicleModelsByUserId(Long userId);
}

我希望在填充vehicle_model数据的可迭代中得到2个结果。相反，我得到了2个结果，但是对于车辆模型属性，我得到了“无法计算表达式方法抛出的'org.hibernate.exception.SQLGramar异常'异常”

以下是控制台的输出：

Answer 1

这是执行我有意想要的操作的解决方案 - 根据user_vehicle表中的user_id搜索车辆：

@Entity(name = "user_vehicle")
public class UserVehicle {

  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private long id;
  private boolean isActive;
  private long userId;

  @ManyToOne
  @JoinColumn(name = "vehicle_id")
  private Vehicle vehicle;


@Entity(name = "vehicle_model")
public class Vehicle {

  @Id
  @Min(1)
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;
  private Long manufacturerId;
  private String title;
  private int ccm;
  private int kw;
  private int yearOfManufacture;
  private int engineTypeId;
  private boolean isActive;

  @OneToMany(mappedBy = "vehicle")
  private Set<UserVehicle> userVehicles;


@Repository
public interface UserVehicleRepository extends CrudRepository<UserVehicle, Long> {

  Iterable<UserVehicle> findVehicleModelsByUserId(Long userId);

User类实际上根本没有使用:

@Entity(name = "user")
public class User {

  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;
  private String username;
  private String password;
  private String firstName;
  private String lastName;
  private boolean isActive;

现在这是一种一对一的关系，并且正在做我第一次真正想要的事情。

希望它能帮助某人

Answer 2

在这里找到了解决方案:https://www.baeldung.com/jpa-many-to-many

我一直在寻找它有点错误，因为这是一个多对多的关系。我的想法是转到“中间”表，选择所有vehicle_iduserId=： id和从vehicle_ids列表中获取每辆车的详细信息。

以下是有效的正确实现：

@Entity(name = "user")
public class User {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String username;
private String password;
private String firstName;
private String lastName;
private boolean isActive;

@OneToMany(mappedBy = "user")
private Set<UserVehicle> userVehicles;

@Entity(name = "user_vehicle")
public class UserVehicle {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private long id;
private boolean isActive;

@ManyToOne
@JoinColumn(name = "vehicle_id")
private Vehicle vehicle;

@ManyToOne
@JoinColumn(name = "user_id")
private User user;


@Entity(name = "vehicle_model")
public class Vehicle {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private Long manufacturerId;
private String title;
private int ccm;
private int kw;
private int yearOfManufacture;
private int engineTypeId;
private boolean isActive;

@OneToMany(mappedBy = "vehicle")
private Set<UserVehicle> userVehicles;

最后，在存储库接口中我有一个这样的方法:

public interface UserVehicleRepository extends CrudRepository<UserVehicle, Long> {

Iterable<UserVehicle> findVehicleModelsByUserId(Long userId);

输出是这样的：

[
    {
        "id": 2,
        "vehicle": {
            "id": 2,
            "manufacturerId": 1,
            "title": "Fazer FZ1S",
            "ccm": 998,
            "kw": 110,
            "yearOfManufacture": 2008,
            "engineTypeId": 1,
            "active": true
        },
        "user": {
            "id": 2,
            "username": "sfajkovic",
            "password": "33",
            "firstName": "Ivan",
            "lastName": "Fajkovic",
            "active": true
        },
        "active": true
    }
]

问题不在于我真的不希望结果中的用户对象，所以现在我需要弄清楚那个。之后，我将尝试映射车辆制造商类，以便在相同的响应中获取这些结果。