__________       ________        ________
   |         |     |hospital|      |        |
   |hospitals|----<| doctor |>-----| doctor |
   |_________|     |________|      |________|

@Entity
@Table(name = "hospital_doctor")
@IdClass(HospitalDoctorId.class)
public class HospitalDoctor {

    @Id
    private int hospital_id;
    @Id
    private int doctor_id;

    @ManyToOne
    @JoinColumn(updatable = false, insertable = false)
    private Doctor doctor;

    @ManyToOne
    @JoinColumn(updatable = false, insertable = false)
    private Hospital hospital;

    @Id
    private int id;
    @Temporal(TemporalType.DATE)
    private Date contract_start_date;
    @Temporal(TemporalType.DATE)
    private Date contract_end_date;
    private String position;
    private String supervisor;
    private boolean part_time; //then getters, setters and HospitalDoctor
                                                              constructors

"exception": "java.lang.NullPointerException",
"message": "No message available",
"path": "/api/hospitaldoctors"

public List<HospitalDoctor> getAll() {
    List<HospitalDoctor> hospitaldoctors = new ArrayList<>();
    hospitalDoctorDao.findAll().forEach(hospitaldoctors::add);
    return hospitaldoctors;
}

@GetMapping(value="/api/hospitaldoctors")
public List<HospitalDoctor> getAll(){
    return hospitalDoctorService.getAll();
}