POST后，使用@ManyToOne关系返回保存的实体

Question

I have a spring boot application with an entity called Person , which has a @ManyToOne relationship:

@Entity
public class Person {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    private String firstName;
    private String lastName;

    @ManyToOne
    private Country country;
    // ...
}

The PersonController is used to allow to GET existing persons and POST new ones:

@RestController
public class PersonController {

    private final PersonRepository personRepository;

    @Autowired
    public PersonController(final PersonRepository personRepository) {
        this.personRepository = personRepository;
    }

    @GetMapping("/person/{personId}")
    public Person getPerson(@PathVariable long personId) {
        return personRepository.findById(personId).get();
    }

    @PostMapping("/person_save_and_return_by_id")
    public Person personSaveAndReturnById(@RequestBody Person person) {
        Person savedPerson = personRepository.save(person);
        // the payload sent by the client does not necessarily include all
        // properties of the Country class, it might contain just the id.
        // Therefore, we fetch the person from the database, so that all
        // necessary JOIN operations are made and no properties are set to null.
        Person samePerson = personRepository.findById(savedPerson.getId()).get();
        return samePerson;
    }
}

I would like to POST new persons without having to describe the existing Country in its entirety, I only want to denote its id:

curl -i -X POST -H "Content-Type:application/json" -d '{"firstName": "Frodo", "lastName": "Baggins", "country": {"id": 1}}' http://localhost:8080/person_save_and_return_by_id

Unfortunately, the returned entity has all country properties, except for the id, set to null:

{
   "firstName" : "Frodo",
   "id" : 4,
   "lastName" : "Baggins",
   "country" : {
      "id" : 1,
      "description" : null,
      "countryCodeIso" : null
   }
}

The POST request inserts everything into the database as it should. For instance, if I GET the object I just previously posted:

curl -i  http://localhost:8080/person/4 

The returned json looks exactly as it should:

{
   "lastName" : "Baggins",
   "firstName" : "Frodo",
   "country" : {
      "id" : 1,
      "countryCodeIso" : "FR",
      "description" : "France"
   },
   "id" : 4
}

But that doesn't help me, because I need to access the Country instance in the same method where the new Person is created.

My question is, why does the newly created person have a country with some properties set to null in the @PostMapping method, when the exact same person does not have those null values in the @GetMapping called right after? How can I access the person's country correctly during the @PostMapping , without resorting to workarounds such as explicitly fetching the country via its id?

The code is also available on github . It uses the h2database, but I have the same issue with hibernate and postgresql. Notice that two countries are inserted during startup, so no foreign key violations are thrown when the country id is set to 1 or 2.

Answer 1

did you try to add

@ManyToOne(cascade=CascadeType.MERGE)

Answer 2

You could fetch only country from the repository given the country_id you provide in the payload. Then set the whole country object to Person and save person.