Java Spring引导中的映射理解问题

我刚开始用弹簧靴。 我有3个表User、UserNotes和Notes

用户（用户ID、电子邮件ID）

UserNotes（UserID，NotesID）

注释（注释SID、标题、消息）

当我添加电子邮件时，用户ID必须自动生成。当我添加与用户对应的任何注释时，UserNotes表中的注释Id必须自动生成，并且必须添加到Notes表中。
在MySQL中，我将User表中的userID作为主键，UserNotes中的userID作为外键引用它。类似地，UserNotes中的NotesID作为主键，而Notes表中的NotesID作为外键

当我使用MySQL查询时，一切正常。但现在我想使用Spring数据JPA来实现这一点。但我在理解如何通过多个表映射这些关系时遇到了困难。我试过一对夫妻和多对夫妻的关系，但都没有成功

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);  

---

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;
}

@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;
}

@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;
}

注释实体

@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;
}

@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;
}

@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;
}

UserNotes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;
}

@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;
}

@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);
}

public interface UserRepository扩展了JpaRepository{
}
公共接口NotesRepository扩展了JpaRepository{
}
公共接口UserNotesRepository扩展了JpaRepository{
列出findByUser（用户）；
}

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());
    }
}

@服务
@交易的
@所需参数构造函数
公共类测试服务{
私有最终用户存储库用户存储库；
私人最终用户备注保留用户备注保留；
公共用户saveUser（用户用户）{
User newUser=新用户（）；
user.setEmail（user.getEmail（））；
返回userRepository.save（用户）；
}
public UserNotes saveNotes（用户用户，Notes）{
UserNotes UserNotes=newusernotes（）；
userNotes.setUser（用户）；
userNotes.setNotes（notes）；
返回userNotesRepository.save（userNotes）；
}
公共列表getNotes（用户）{
return userNotesRepository.findByUser（用户）
.stream（）
.map（UserNotes:：getNotes）
.collect（Collectors.toList（））；
}
}

您应该了解不同类型的实体关系，特别是许多关系。连接表UserNote不会生成任何内容，因为它们是它包含的外键，必须首先在相应的表中创建它们。一般来说，我建议您阅读一本关于sql的书，因为您需要了解一些基础知识。