[FIXED] Spring Boot: POST request to entity with ManyToMany relationship

Issue

I'm working on a database for adding bands, musicians, instruments, etc.

I have a table 'band' and a table 'musician'. They have a ManyToMany relationship (one band can have many musicians, a musician can be in many bands), with an extra table BandMusician that has an embeddedId BandMusicianId. I did it like this because I want the relationship between bands and musicians to have also other information, like the year the musician joined the band.

@Entity
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Band {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    private String genre;
    private int year;

    @OneToOne(mappedBy = "band")
    private Website website;

    @OneToMany(mappedBy = "band")
    private List<Album> albuns;

    @OneToMany(mappedBy = "band")
    private List<BandMusician> musicians;

}


@Entity
@Data
@AllArgsConstructor
@NoArgsConstructor
@JsonDeserialize(using = MusicianJsonDeserializer.class)
public class Musician {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    @JsonFormat(pattern = "dd-MM-yyyy")
    @JsonProperty("DoB")
    @Column(name = "date_of_birth")
    private LocalDate DoB;

    @ManyToMany
    @JoinTable(
            name = "musician_instruments",
            joinColumns = @JoinColumn(name = "musician_id"),
            inverseJoinColumns = @JoinColumn(name = "instrument_id")
    )   
    private List<Instrument> instruments = new ArrayList<>();

    @OneToMany(mappedBy = "musician")
    private List<BandMusician> bands;

    public void addInstrument(Instrument instrument) {
        this.instruments.add(instrument);
    }

}


@Embeddable
@Data
@AllArgsConstructor
@NoArgsConstructor
public class BandMusiciansId implements Serializable{
    
    private static final long serialVersionUID = 1L;

    @Column(name = "band_id")
    private Long bandId;

    @Column(name = "musician_id")
    private Long musicianId;
}


@Entity
@Data
@AllArgsConstructor
@NoArgsConstructor
public class BandMusician {

    @EmbeddedId
    private BandMusiciansId id = new BandMusiciansId();

    @ManyToOne
    @MapsId("bandId")
    @JoinColumn(name = "band_id")
    private Band band;

    @ManyToOne
    @MapsId("musicianId")
    @JoinColumn(name = "musician_id")
    private Musician musician;

    private String role;
    private int joined;
}

When I receive a POST request to "/musician" I can save a musician. I'm using Jackson to deserialize a request like this:

{
    "name": "John the Ripper",
    "DoB": "03-12-1965",
    "instruments": "voice, guitar",
    "bands": "Band1, Band2"
}

With Jackson I can get each band, search with the BandRepository and create a BandMusician.

THE PROBLEM: When I receive the request, in order to create a BandMusician I have to create a BandMusiciansId, and to do that I need the bandId and the MusicianId. But I'm creating the musician right now, so I don't have the musicianId. It is created automatically when I save the musician.

MusicianJsonDeserializer class

public class MusicianJsonDeserializer extends JsonDeserializer<Musician>{

private final InstrumentRepository instrumentRepository;
private final BandRepository bandRepository;

@Autowired
public MusicianJsonDeserializer(
        InstrumentRepository instrumentRepository,
        BandRepository bandRepository
) {
    this.instrumentRepository = instrumentRepository;
    this.bandRepository = bandRepository;
}

@Override
public Musician deserialize(JsonParser p, DeserializationContext ctxt) 
        throws IOException, JacksonException {
    
    ObjectCodec codec = p.getCodec();
    JsonNode root = codec.readTree(p);
    
    Musician musician = new Musician();
    musician.setName(root.get("name").asText());
    
    DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd-MM-yyyy");
    musician.setDoB(LocalDate.parse(root.get("DoB").asText(), formatter));
    
    if (root.get("instruments") != null) {
        String instrumentList = root.get("instruments").asText();
        String[] instrumentArray = instrumentList.split(", ");
        List<Instrument> musicianInstrumentList = new ArrayList<>();
        
        for (String instrument : instrumentArray) {
            Instrument instrumentFound = 
                    instrumentRepository.findByName(instrument)
                    .orElseThrow(RuntimeException::new);
            // TODO custom exception
            musicianInstrumentList.add(instrumentFound);
        }
        
        musician.setInstruments(musicianInstrumentList);
    }
    
    if (root.get("bands") != null) {
        // TODO Stuck here!

What I thought of doing: In my MusicianService, after saving the musician, I can create the BandMusician and the relationship. I think doing this in the Service layer would be a bad choice though.

EDIT: To make it easier to understand, I created a project only with the relevant parts of this one and pushed to github (https://github.com/ricardorosa-dev/gettinghelp). Again, what I want is to be able to send a POST to "/musician", that will be caught by the MusicianJsonDeserializer, and somehow create a BandMusicianId and BandMusician for each band sent in the request body.

Solution

I have the entities Band and Musician and a ManyToMany relationship between them with an association table BandMusician.

What I wanted was to create the entity Musician and the relationship (BandMusician) in the same request.

As far as I can gather it is not possible, because in order to create a record in the association table (BandMusician), I would have to have the musician (I'm creating in this request) already created.

I tried everything just to see if it was POSSIBLE and wasn't able to do it. But even if it was possible, it would be a very bad practice, since it would make the class too tightly coupled.

The clear solution was to create only the Musician with this request, and then send another request to create the connection between Band and Musician.

I also tried to create many entries in the BandMusician table with one request, which was also impossible, because the JsonDeserializer table doesn't seem to accept List<> as a return type. I was trying to avoid making a lot of requests to create the relationship entries (for a musician that is in five bands, for example), but it seems it is better to keep things clear and simple.

I now save one musician-band relationship per request:

{
    "musician": "Awesome musician",
    "band": "Awesome band",
    "role": "guitar",
    "joined": 2003
}

Answered By - ricardorosa-dev