Best way to handle negative scenario in spring boot - spring-boot

Question

So I am working on a spring-boot application for a zoo. while adding animals I can assign some room and mark some rooms as favorites, My sample JSON looks like below
{
"id":null,
"title":null,
"located":null,
"type":null,
"preference":null,
"favoriteRooms":null,
"room":{
"id":null,
"title":"Green",
"size":25,
"createdOn":null,
"favorites":null,
"animals":null
}
}
now I want to make sure the room should be valid while adding animals, if there is no room available I want to throw an error. currently, I am using CascadingType.MERGE but it throws hibernate exception I want to do some valid addition for room and favorite room what is the best way to do this?
my entity class looks like below
#Entity
#Data
#NoArgsConstructor
#FieldDefaults(level = AccessLevel.PRIVATE)
public class Animal {
#Id
#GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "seq_animal_id")
#SequenceGenerator(name = "seq_animal_id")
Long id;
#NotEmpty(message = "title should be given")
String title;
#CreatedDate
#JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd'T'HH:mm:ss")
LocalDateTime located;
String type;
Long preference;
#OneToMany(mappedBy = "animal")
Set <Favorite> favoriteRooms;
#ManyToOne(cascade = CascadeType.MERGE)
#JoinColumn(name = "room_id")
Room room;
}

Answer 1

I'm not sure if I really understand what you're asking for but you can handle this in your controller or better in ServiceImplementation which handles the business logic.
Using #Valid the constraints will be checked which you have defined in your models:
example:
#Autowired
RoomRepository roomRepository;
#PostMapping
public ResponseEntity<?> insertAnimal(#Valid #RequestBody Animal animal) {
Optional<Room> optionalRoom = roomRepository.findById(animal.getRoom.getRoomId());
if (optionalRoom.isPresent()) {
Room room = optionalRoom.get();
// your logic to check if the room is valid
}
else {
return new ResponseEntity<String>("Your message here", HttpStatus.NOT_FOUND);
}
}