Spring Boot双向OneToMany关联POST请求中Playlist的user字段为null问题解决

在实现Spring Boot双向OneToMany关联时，创建用户同时传入新Playlist数据，返回的Playlist对象中user字段为null，无法建立内存中的双向关联。

User实体类 #

@Entity
@Table(name = "users")
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id ;

    private String username ;
    private String password ;
    private String email ;
    private String firstname ;
    private String lastname ;

    @OneToMany(
            mappedBy = "user" ,
            cascade = CascadeType.ALL ,
            orphanRemoval = true
    )
    private List<Playlist> playlists = new ArrayList<>() ;

    // getters and setters
}

Playlist实体类 #

@Entity
@Table(name = "playlist")
public class Playlist {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;
    private String description;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "user_id")
    private User user;

    // getters and setters

}

UserController的POST方法 #

@RestController
@RequestMapping("/api/users")
public class userController {

    @Autowired
    UserService userService ;

    // ... other controller mappings

    @PostMapping("")
    public User createUser(@RequestBody User reqUser){
        return userService.createUser(reqUser) ;
    }
}

UserService的createUser方法 #

@Service
public class UserService {

    @Autowired
    UserRepo userRepo ;

    public List<User> getAllUsers() {
        List <User> users = new ArrayList<>() ;
        userRepo.findAll().forEach(users::add);
        return users ;
    }

    public User createUser(User reqUser) {
        User newUser = new User() ;
        newUser.setUsername(reqUser.getUsername());
        newUser.setEmail(reqUser.getEmail());
        newUser.setPassword(reqUser.getPassword());
        newUser.setFirstname(reqUser.getFirstname());
        newUser.setLastname(reqUser.getLastname());

        // adding playlists 
        List<Playlist> playlists = new ArrayList<>() ;
        reqUser.getPlaylists().forEach(playlists::add);
        newUser.setPlaylists(playlists);

        userRepo.save(newUser) ;

        return newUser ;
    }
}

请求JSON #

{
    "username" : "poodie" ,
    "password" : "randomPasswrod" ,
    "firstname" : "Raj" ,
    "lastname" : "Bhushan" ,
    "playlists" : [
        {
            "name" : "playlist-01" ,
            "description" : "first playlist" 
            
        } ,

        {
            "name" : "playlist-01" ,
            "description" : "first playlist" 
        }
    ]
}

响应JSON #

{
    "id": 12,
    "username": "poodie",
    "password": "randomPasswrod",
    "email": null,
    "firstname": "Raj",
    "lastname": "Bhushan",
    "playlists": [
        {
            "id": 9,
            "name": "playlist-01",
            "description": "first playlist",
            "user": null,
            "songs": []
        },
        {
            "id": 10,
            "name": "playlist-02",
            "description": "first playlist",
            "user": null,
            "songs": []
        }
    ]
}

问题原因 #

在双向OneToMany关联中，@OneToMany的mappedBy = "user"指定了Playlist是关联的维护端（即外键user_id由Playlist实体管理）。当前代码只设置了User的playlists集合，但没有给每个Playlist对象设置对应的user属性，导致内存中的Playlist对象和User对象的双向关联没有建立。虽然级联保存会把User的id写入Playlist的user_id字段（数据库中关联是正确的），但返回的是保存前构造的内存对象，其中Playlist的user字段为null。

修复方案1：维护双向关联的一致性（推荐） #

在User实体中添加辅助方法，用于添加Playlist时自动设置关联的User，确保双向关联同步：

@Entity
@Table(name = "users")
public class User {
    // ... 原有字段

    @OneToMany(
            mappedBy = "user" ,
            cascade = CascadeType.ALL ,
            orphanRemoval = true
    )
    private List<Playlist> playlists = new ArrayList<>() ;

    // 添加辅助方法
    public void addPlaylist(Playlist playlist) {
        playlists.add(playlist);
        playlist.setUser(this); // 关键：设置Playlist的user属性
    }

    // getters and setters
}

修改UserService中的createUser方法，使用这个辅助方法添加Playlist：

public User createUser(User reqUser) {
    User newUser = new User() ;
    newUser.setUsername(reqUser.getUsername());
    newUser.setEmail(reqUser.getEmail());
    newUser.setPassword(reqUser.getPassword());
    newUser.setFirstname(reqUser.getFirstname());
    newUser.setLastname(reqUser.getLastname());

    // 改用addPlaylist方法添加
    for (Playlist playlist : reqUser.getPlaylists()) {
        newUser.addPlaylist(playlist);
    }

    userRepo.save(newUser) ;

    return newUser ;
}

这样保存后，内存中的Playlist对象就会持有正确的User引用，返回的JSON中user字段就不会为null。

修复方案2：保存后从数据库重新查询 #

如果不想修改实体类，也可以在保存User后，从数据库中重新查询该User返回。因为数据库中的关联关系已经正确建立，查询出来的对象会自动填充Playlist的user属性：

public User createUser(User reqUser) {
    User newUser = new User() ;
    newUser.setUsername(reqUser.getUsername());
    newUser.setEmail(reqUser.getEmail());
    newUser.setPassword(reqUser.getPassword());
    newUser.setFirstname(reqUser.getFirstname());
    newUser.setLastname(reqUser.getLastname());

    List<Playlist> playlists = new ArrayList<>() ;
    reqUser.getPlaylists().forEach(playlists::add);
    newUser.setPlaylists(playlists);

    // 保存后重新查询
    User savedUser = userRepo.save(newUser) ;
    return userRepo.findById(savedUser.getId()).orElse(savedUser);
}

内容的提问来源于stack exchange，提问作者humblefool