Hibernate中有效的更新实体

Question

我想这可能是一个新手问题，但我仍然想知道一些答案。

假设存在实体：医院和医生（多对多）。假设在我的controller类中，我必须获取所有现有的医生和医院，然后在特定的医院雇佣一名医生

@Controller
public class HospitalController {
  ...
    @RequestMapping("/hireDoctor")
    public String (HttpServletRequest request, Model model) {
        List<Hospital> hospitals = hospitalService.findAllHospitals();
        List<Doctor> doctors = doctorService.findAllDoctors();

        //some logic, in the end we choose just one doctor and one hospital

        Doctor doctor = doctors.get(0);
        Hospital hospital = hospitals.get(0);

        hospitalService.hireDoctor(hospital, doctor);
    }
  ...

@Service
public class HospitalService {
  ..  
    @Transactional
    public void hireDoctor(Hospital hospital, Doctor doctor) {
        //ideally
        List<Doctor> doctors = hospital.getDoctors();
        doctors.add(doctor);

        this.em.merge(hospital);
    }
  ..
}

它当然不起作用，因为--据我所知--我已经在控制器中获取了所有的医生和医院，然后在hireDoctor方法中，我们打开了传递常规Java对象的trasaction，这些对象不在会话中。

我知道，我可以用一个特殊的身份证再次去医院，用一个特定的身份证去医生，然后保存它

public void hireDoctor(Hospital hospital, Doctor doctor) {
     Hospital h = hospitalRepo.getHospitalById(hospital.getId());
     Doctor d = hospitalRepo.getDoctorById(doctor.getId());
     List<Doctor> doctors = h.getDoctors();
     doctors.add(d);
}

但看起来就是垃圾。

Answer 1

有一个很好和优雅的方法来做到这一点。它依赖于使用Hibernate代理，并将多对多关系提取到一个单独的实体，例如：

@Entity
public class HospitalToDoctor implements Serializable {
    @Id
    @ManyToOne
    private Hospital hospital;

    @Id
    @ManyToOne
    private Doctor doctor;
}

@Entity
public class Hospital {
    @OneToMany(mappedBy = "hospital")
    private Collection<HospitalToDoctor> doctors;
}

@Entity
public class Doctor {
    @OneToMany(mappedBy = "doctor")
    private Collection<HospitalToDoctor> hospitals;
}

现在，为了只使用一条insert语句而不进行任何额外的数据库往返，来分析Doctor和Hospital:

HospitalToDoctor hospitalToDoctor = new HospitalToDoctor();
hospitalToDoctor.setHospital(entityManager.getReference(Hospital.class, hospitalId));
hospitalToDoctor.setDoctor(entityManager.getReference(Doctor.class, doctorId));
entityManager.persist(hospitalToDoctor);

这里的关键点是使用EntityManager.GetReference：

public Collection<Doctor> getDoctors() {
    Collection<Doctor> result = new ArrayList<>(doctors.size());
    for (HospitalToDoctor hospitalToDoctor : doctors) {
        result.add(hospitalToDoctor.getDoctor());
    }
    return result;
}

但是，如果您仍然不想引入一个单独的实体，而是想使用一个干净的Hibernate多对多，您仍然可以从代理中受益。您可以将Doctor代理添加到加载的Hospital（反之亦然）。您可能还希望查看Hibernate额外的惰性集合，以避免在将Doctor添加到Hospital或反之亦然时加载Doctors集合（我想这是您问题的主要关注点）。