我使用的是springboot和mysql。我尝试在播放列表表中添加新实体（歌曲）。他们有多对多的关系。但正如您在mysql查询后的回答中看到的，它并没有保存。其他关系正常
播放列表实体

@Data
@Entity
@Component
@Getter
@EqualsAndHashCode(exclude = "songs")
@NoArgsConstructor
@AllArgsConstructor
@Table(name = "playlists")
public class PlaylistEntity {
    @Id
    @GeneratedValue(generator = "uuid")
    @GenericGenerator(name = "uuid", strategy = "uuid2")
    private String id;

    private String playlistTitle;

    @ManyToOne(fetch = FetchType.EAGER, cascade = {CascadeType.MERGE})
    @JoinColumn(name = "user_id", nullable = false)
    private UserEntity user;

    private LocalDateTime createdAt;

    public PlaylistEntity(String playlistTitle, UserEntity user, LocalDateTime createdAt) {
        this.playlistTitle = playlistTitle;
        this.user = user;
        this.createdAt = createdAt;
    }

    @Transient
    @ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    @JoinTable(name = "playlist_song",
            joinColumns = @JoinColumn(name = "playlist_id", nullable=false),
            inverseJoinColumns = @JoinColumn(name = "song_id", nullable=false))
    private Set<SongEntity> songs = new HashSet<>();

}

播放列表存储库

@Repository
public interface PlaylistRepository extends PagingAndSortingRepository<PlaylistEntity, String> {
    @Query(value = "select * from playlists where user_id = :id", nativeQuery = true)
    List<PlaylistEntity> showAllUserPlaylists(@Param("id") String id);

    @Query(value = "select * from playlists where playlist_title = :playlist_title", nativeQuery = true)
    PlaylistEntity findByName(@Param("playlist_title") String playlist_title);
}

宋体

@Data
@Entity
@Builder
@Getter
@EqualsAndHashCode(onlyExplicitlyIncluded = true)
@NoArgsConstructor
@AllArgsConstructor
@Table(name = "songs")
public class SongEntity {

    @Id
    @GeneratedValue(generator = "uuid")
    @GenericGenerator(name = "uuid", strategy = "uuid2")
    private String id;

    private String title;

    private String artist;

    private String album;

    private String genre;

    @CreationTimestamp
    private LocalDateTime releaseDate;

    private int likes;

    @Transient
    @EqualsAndHashCode.Exclude
    @ManyToMany(mappedBy = "songs")
    private Set<PlaylistEntity> playlistEntities = new HashSet<>();

    @Transient
    @ManyToMany(mappedBy = "songs")
    @EqualsAndHashCode.Exclude
    private Set<SubscriptionEntity> subscriptionEntities = new HashSet<>();

    public SongEntity(String name) {
        this.title = name;
    }
}

歌曲库

@Repository
public interface SongRepository extends JpaRepository<SongEntity, String> {

    @Query(value="SELECT * FROM songs WHERE (:genre is null or genre = :genre) " +
            "AND (:artist IS NULL or artist = :artist)", nativeQuery=true)
    List<SongEntity> findByParams(@Param("genre") String genre, @Param("artist") String artist);

    @Query(value="SELECT * FROM songs WHERE artist = :artist", nativeQuery=true)
    List<SongEntity> findByArtist(@Param("artist") String artist);

    @Query(value="SELECT * FROM songs WHERE genre = :genre", nativeQuery=true)
    List<SongEntity> findByGenre(@Param("genre") String genre);

    @Query(value = "SELECT s.title, s.likes FROM SongEntity s WHERE s.artist = :artist")
    List<SongEntity> showSongsStatistics(@Param("artist") String artist);
}

在播放列表表中保存歌曲的方法

@Transactional
public Playlist addSongToPlaylist(String playlistId, String songId) throws Exception {

    SongEntity addedSong = findSongById(songId)
    PlaylistEntity requiredPlaylist = findPlaylistById(playlistId);

    requiredPlaylist.getSongs().add(addedSong);

    PlaylistEntity updatedPlaylist = playlistRepository.save(requiredPlaylist);

    return playlistConverter.fromEntity(updatedPlaylist);
}

和控制器

@Slf4j
@Configuration
@RestController
@AllArgsConstructor
@RequestMapping("/user/playlists")
public class PlaylistController {

    private final PlaylistService playlistService;

    @PostMapping(value = ADD_SONG_TO_PLAYLIST_URL)
    Playlist addSongToThePlaylist(@RequestParam String playlistId, @RequestParam String songId) throws Exception {
        return playlistService.addSongToPlaylist(playlistId, songId);
    }

    @UtilityClass
    public static class Links {
        public static final String ADD_SONG_TO_PLAYLIST_URL = "/addSong";
    }
}

我用 Postman 提出请求。在请求之后，我得到了这个答案，这表明这首歌被添加到了播放列表中。https://i.stack.imgur.com/vdfbc.png
但正如我所说，如果检查播放列表的歌曲数据库，它什么都没有。这意味着我的程序不能正确地保存多对多表。https://i.stack.imgur.com/9lgw9.png
日志中也有例外。
所以我能理解什么是错的。希望有人有主意。