Попытка POST через POSTMAN json данные с внешним ключом

Question

У меня есть две сущности, myAppUser и Absences, и я пытаюсь создать Absence, связанную с его «владельцем», пользователем. Отсутствия опубликованы хорошо, но связь между отсутствием и пользователем никогда не происходит.

@Entity
public class Absence {

    @GeneratedValue(strategy=GenerationType.IDENTITY)   
    @Id
    @JsonView(View.Summary.class)
    private int id;
    @Nullable
    @JsonView(View.SummaryWithOthers.class)
    private Date dateMissing; 

    @Nullable
    @JsonProperty(access = Access.READ_WRITE)
    @JsonView(View.SummaryWithOthers.class)
    private String comment; 

    @ManyToOne (fetch = FetchType.EAGER)
    @JsonView(View.Summary.class)
    @JoinColumn (name="student_id", nullable = false)
    private MyAppUser userStudent;

@Entity
@Table(name="users")
public class MyAppUser {

    //@GeneratedValue(strategy=GenerationType.IDENTITY) 
    @Id
    @JsonView(View.Summary.class)
    private String id;
    @JsonView(View.Summary.class)
    private String firstName;
    @JsonView(View.Summary.class)
    private String lastName;
    @JsonView(View.SummaryWithOthers.class)
    private String email;
    @JsonView(View.SummaryWithOthers.class)
    private char gender;
    @JsonView(View.SummaryWithOthers.class)
    private int age;
    @JsonView(View.SummaryWithOthers.class)
    private String portrait;
    @JsonView(View.Summary.class)
    @ManyToOne
    private Seat seat;

    @JsonView(View.SummaryWithOthers.class)    
    @ManyToOne
    private Absence userAbsence;

    @ManyToOne (fetch = FetchType.EAGER)
    @JoinColumn (name="rol_id")
    private Role role;

    private String password;
    private boolean enabled;
    private Date lastLogin;

    @OneToMany (targetEntity = Absence.class, cascade = CascadeType.ALL)
    private List <Absence> absences = new ArrayList <Absence>();
    @OneToMany (targetEntity = Course.class, cascade = CascadeType.ALL)
    @JsonView(View.Summary.class)
    public List <Course> courses = new ArrayList <Course>();
    @OneToMany (targetEntity = UserExercice.class, cascade = CascadeType.ALL)
    private List <UserExercice> userExercices = new ArrayList <UserExercice>();
    @OneToMany (targetEntity = Emails.class, cascade = CascadeType.ALL)
    private List <Emails> emails = new ArrayList <Emails>();

    @OneToMany(mappedBy="myAppUser")
    @JsonIgnore
    Set<UserIteration> userIterations;

MyAppUser Repository

public interface MyAppUserRepository extends JpaRepository<MyAppUser, String>{

    public ArrayList<MyAppUser> findByFirstName(String firstName);
    public ArrayList<MyAppUser> findByLastName(String lastName);
    public List<MyAppUser> findByGender(char gender);
    public ArrayList<MyAppUser> findByRoleId(int roleId);

    Optional <MyAppUser> findOneById(String id );

    MyAppUser findUserById(String id );
}

Контроллер

    //Create absence by student id
    @PostMapping("/api/students/absences")
    public Absence createAbsence(@RequestBody Absence absence, MyAppUser userStudent) {
        return absencesService.createAbsence(absence, userStudent);
    }

Сервис

    public Absence createAbsence(Absence absence, MyAppUser userStudent) {

        String studentId = userStudent.getId(); //absence.getUserStudent().getId();
        Absence absenceCreated = new Absence();
        userStudent = myAppUserRepository.findUserById(studentId);

        absenceCreated.setComment(absence.getComment());
        absenceCreated.setDateMissing(absence.getDateMissing());
        absenceCreated.setUserStudent(userStudent);

        myAbsenceRepository.save(absenceCreated);
        return absenceCreated;
    }

JSON в POSTMAN

{
    "userStudent":{
            "student_id":"58a55a41-04cc-4007-a322-e7bb969e626b"
        },
    "dateMissing": "1574541388697",
    "comment": "prueba"
}

И он возвращает это:

{
    "id": 485,
    "dateMissing": 1574541388697,
    "comment": "medic issue",
    "userStudent": null
}

Я не знаю, набираю ли я JSON неправильно, или если мой сервис работает неправильно, но я по-настоящему noob в Spring и не понимаю ... Просто нужно получить доступ к репозиторию пользователя и получить его по его String ID. Как я могу?

Благодарю вас !! ^^