JPA在刷新之前保存临时实例

Question

I have the following situation: a student belongs to a team, a team can have many students. At registration time the student doesn't know the assigned team, so the team object in student class should be null. How to force the insertion of student with null team object without getting this error:

org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing : com.af.domain.Student.team -> com.af.domain.Team

I tried cascade=CascadeType.ALL but it also ads a new team in the database.

Student class

@Entity
@Table(name = "student")
public class Student implements DomainModel{

    @Id
    @GeneratedValue
    private Integer idStudent;
    private String firstname;
    private String lastname;
    private String username;
    private String password;
    @ManyToOne
    @JoinColumn(name="idTeam", nullable=true)
    private Team team;  
}

Team class

@Entity
@Table
public class Team implements DomainModel{
    @Id
    @GeneratedValue
    private Integer idTeam;
    private String teamName;
    @OneToMany(mappedBy="team")
    private Set<Student> students;
}

Persistence class

public class GenericDAOImpl<T extends DomainModel> implements GenericDAO<T>  {
    public void save(T object) {
       EntityManager entityManager = entityManagerFactory
            .createEntityManager();
       entityManager.getTransaction().begin();
       try {
          entityManager.persist(object);
       } catch(NullPointerException ex){
          ex.printStackTrace();
       }finally {
          entityManager.getTransaction().commit();
          entityManager.close();
       }
    }
}

Answer 1

You can use detach option with entity manager. entityManager.detach --- This will detach that particular parent object from the main entity object which you want to persist or merge.