Jpawithoutentityrelationship

gi— layout: post title: JPA without entity relationships description: category: JPA tags:

• JPA

Intro

JPA without 연관관계 (entity relationships) like @ManyToOne ultimately cannot product object-orientated code. This is because we search up entities and update them on their primary keys.

Without 연관관계

Let’s look at this example

@Entity
public class Patient{
    @Id @GeneratedValue
    @Column(name="patient_id")
    private Long id;
    
    private String name;
    
    //remember JPA (latest versions) automatically changes
    //java's camelCase name to preferred DB naming convention
    //hospitalId -> hospital_id
    private Long hospitalId;
}

@Entity
public class Hospital{
    @Id @GeneratedValue
    @Column(name = "hospital_id")
    private Long id;
    
    private String name;
}

Without 연관관계, we need to use FK of entities.

EntityManagerFactory emf = Persistence.createEntityManagerFactory("hokla");
EntityManager em = emf.createEntityManager();

EntityTransaction tx = em.getTransaction();
tx.begin();

try{
    Hospital hospital = new Hospital();
    hospital.setName("Hospital Hola");
    em.persist(hospital);

    Patient patient = new Patient();
    patient.setName("Wally");
    patient.setHospitalId(hospital.getId());
    em.persist(patient);
    tx.commit();
  } catch (Exception e){
    tx.rollback();
  } finally {
    em.close();
}

This is not very object-oriented because we optimally want to somehow get patient.getHosptial() like getting objects, not FKs.

With 연관관계

Let’s add @ManyToOne to map entities’ relationship

@Entity
public class Patient{
    @Id @GeneratedValue
    @Column(name="patient_id")
    private Long id;
    
    private String name;
    
    //remember JPA (latest versions) automatically changes
    //java's camelCase name to preferred DB naming convention
    //hospitalId -> hospital_id
    @ManyToOne
    private Long hospitalId;
}

@Entity
public class Hospital{
    @Id @GeneratedValue
    @Column(name = "hospital_id")
    private Long id;
    
    private String name;
}

Then we can call it the OO way:

EntityManagerFactory emf = Persistence.createEntityManagerFactory("hokla");
EntityManager em = emf.createEntityManager();

EntityTransaction tx = em.getTransaction();
tx.begin();

try{
    Hospital hospital = new Hospital();
    hospital.setName("Hospital Hola");
    em.persist(hospital);

    Patient patient = new Patient();
    patient.setName("Wally");
    patient.setHospital(hospital);
    em.persist(patient);
    tx.commit();
  } catch (Exception e){
    tx.rollback();
  } finally {
    em.close();
}