[FIXED] Data is being sent to database as null after sending from frontend

Issue

I am trying to make a connection between an Event and User entity, so that I can save entrants into an event. I have never used @ManyToMany mapping before and so have been following a tutorial. When I try to post the data via postman (eventid and userid), I get null values for both.

So far I have, User and Event entity,

@Data
//Entity maps object to database
@Entity
@NoArgsConstructor
@Table(name = "member")
public class User implements UserDetails, Serializable {

//More fields

//Relationship between user and events to get entrants
    @OneToMany(mappedBy = "userid", fetch = FetchType.LAZY)
    Set<Entrants> entrants;

@Data
//Entity maps object to database
@Entity
@NoArgsConstructor
@Table(name = "event")
public class Event implements Serializable {

//More fields

//Relationship with event and users for entrants to an event
    @OneToMany(mappedBy = "eventid",fetch = FetchType.LAZY)
    Set<Entrants> entrants;

Then I have an Entrant Entity to hold the entrants to an event.

@Entity
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Entrants implements Serializable {

    @Id
    @GeneratedValue
    Long id;

    @ManyToOne
    @JoinColumn(name = "user_id")
    User userid;

    @ManyToOne
    @JoinColumn(name = "event_id")
    Event eventid;


}

Then in my controller,

@PostMapping("/management/events/entrants")
    GenericResponse createEntrant(@Valid @RequestBody Entrants entrant) {
        System.out.println("entrant is: " +entrant);
        entrantService.save(entrant);
        return new GenericResponse("Entrant saved");
    }

EntrantService

public Entrants save(Entrants entrants) {
        return entrantRepository.save(entrants);
    }

and the repository is the standard and the above utilises the save() method.

If I post the following in Postman,

{
    "user_id": 1,
    "event_id": 1
}

I get this entrant is: Entrants(id=null, userid=null, eventid=null) id is obviously created by Spring, but the userid and eventid are null.

From my limited knowledge I think this is something to do with the 2 fields in the Entrants entity, being of type User and Event rather than int. But I am not sure how to get around this.

The tutorial I followed wasnt really based on my implementation so I have had to change quite a lot.

Solution

You could use a Dto in your controller like this:

@PostMapping("/management/events/entrants")
   createEntrant(@Valid @RequestBody EntrantDto entrant) {
        System.out.println("entrant is: " +entrant);
        entrantService.save(entrant);
        return new GenericResponse("Entrant saved");
    }

EntrantDto.java

public class EntrantDto {
    private Long user_id;
    private Long event_id;
    // no-args constructor, getter, setter,...
}

and modify a little bit your service like

public Entrants save(EntrantDto entrant) {
    User user = this.userRepository.findById(entrant.getUser_id()).orElseThrown(IllegalArgumentException::new);
    Event event = this.eventRepository.findById(entrant.getEvent_id()).orElseThrown(IllegalArgumentException::new);
    Entrants entrants = new Entrants(user, event);
    return entrantRepository.save(entrants);
}

Answered By - Linh Vu