java - 在 Java Spring boot 中理解映射的问题

Question

我刚刚开始使用 Spring Boot。我有 3 个表 User、UserNotes 和 Notes。

1. 用户（用户 ID、电子邮件 ID）
2. 用户注释（用户 ID、注释 ID）
3. 注释（notesID、标题、消息）

当我添加电子邮件时，必须自动生成 UserID。当我添加与用户对应的任何注释时，必须自动生成UserNotes表中的 Notes Id，并且必须将其添加到Notes表中。
我在 MySQL 中通过将User表中的 userID 作为主键并将 UserNotes 中的 UserID 作为引用它的外键来完成它。类似地，UserNotes中的 NotesID作为主键，Notes表中的 notesID 作为外键。

当我为此使用 MySQL 查询时，一切正常。但现在我想为此使用 Spring Data JPA。但是我在理解如何映射具有多个表的这些关系方面遇到了困难。我尝试过OneToMany和ManyToOne关系，但没有成功

MySQL Queries for the reference  
1) Create table User(userID int AUTO_INCREMENT, emailID varchar(40), PRIMARY KEY(userID));  
2) Create table UserNotes(userID int, NotesID int AUTO_INCREMENT, PRIMARY KEY(NotesID), Foreign key(UserID) references User(UserID) on DELETE CASCADE);  
3) Create table Notes(notesID int, title varchar(100), message varchar(500), Date date, PRIMARY KEY(notesID), FOREIGN KEY(notesID) references UserNotes(NotesID) on DELETE CASCADE);  

标签： javamysqlspringspring-boot

Answer 1

使用 Hibernate、JPA、Lombok 的未经测试的示例：

用户实体

@Entity
@Table(name = "USER")
@SequenceGenerator(name = "userSeqId", sequenceName = "user_seq_id", allocationSize = 1)
@NoArgsConstructor
@Setter
@Getter
public class User {

    @Id
    @GeneratedValue(generator = "userSeqId", strategy = GenerationType.AUTO)
    private Long id;

    @NotBlank
    @Column(name = "EMAIL", unique = true)
    private String email;
}

笔记实体

@Entity
@Table(name = "NOTES")
@SequenceGenerator(name = "notesSeqId", sequenceName = "notes_seq_id", allocationSize = 1)
@NoArgsConstructor
@Setter
@Getter
public class Notes {

    @Id
    @GeneratedValue(generator = "notesSeqId", strategy = GenerationType.AUTO)
    private Long id;

    @NotBlank
    @Column(name = "TITLE")
    private String title;

    @NotBlank
    @Column(name = "MESSAGE")
    private String message;
}

UserNotes 实体

@Entity
@Table(name = "USER_NOTES")
@NoArgsConstructor
@Setter
@Getter
public class UserNotes {

    @EmbeddedId
    private UserNotesKey id;

    @NotNull
    @ManyToOne
    @MapsId("USER_ID")
    @JoinColumn(name = "USER_ID")
    @OnDelete(action = OnDeleteAction.CASCADE)
    private User user;

    @NotNull
    @ManyToOne(cascade = CascadeType.PERSIST)
    @MapsId("NOTES_ID")
    @JoinColumn(name = "NOTES_ID")
    @OnDelete(action = OnDeleteAction.CASCADE)
    private Notes notes;
}

@Embeddable
@NoArgsConstructor
@AllArgsConstructor
@Setter
@Getter
@EqualsAndHashCode
public class UserNotesKey implements Serializable {

    @Column(name = "USER_ID")
    private Long userId;

    @Column(name = "NOTES_ID")
    private Long notesId;
}

存储库

public interface UserRepository extends JpaRepository<User, Long> {

}

public interface NotesRepository extends JpaRepository<Notes, Long> {

}

public interface UserNotesRepository extends JpaRepository<UserNotes, UserNotesKey> {

    List<UserNotes> findByUser(User user);
}

测试服务

@Service
@Transactional
@RequiredArgsConstructor
public class TestService {

    private final UserRepository userRepository;
    private final UserNotesRepository userNotesRepository;

    public User saveUser(User user) {
        User newUser = new User();
        user.setEmail(user.getEmail());
        return userRepository.save(user);
    }

    public UserNotes saveNotes(User user, Notes notes) {
        UserNotes userNotes = new UserNotes();
        userNotes.setUser(user);
        userNotes.setNotes(notes);
        return userNotesRepository.save(userNotes);
    }

    public List<Notes> getNotes(User user) {
        return userNotesRepository.findByUser(user)
                .stream()
                .map(UserNotes::getNotes)
                .collect(Collectors.toList());
    }
}