一個與同一實體很多關係

Question

請在下面找到實體代碼，

@Entity 
public class A implements Serializable { 

    @Id 
    @GeneratedValue(strategy=GenerationType.AUTO) 
    private Long id; 

    @OneToMany(cascade={CascadeType.PERSIST, CascadeType.MERGE}, fetch = FetchType.LAZY, mappedBy="parentActivity")   
    private Set<A> subActivities; 

    @ManyToOne(cascade = CascadeType.REFRESH, fetch = FetchType.LAZY) 
    @JoinColumn(name = "PARENTACTIVITYID", insertable = true, updatable = true) 
    private A parentActivity; 

    // Getters, Setters, serialVersionUID, etc... 
} 

如果我們想都在同一時間，然後下面的代碼堅持家長和孩子

運行完美

public static void main(String[] args) { 

    EntityManager em = ... // from EntityManagerFactory, injection, etc. 

    em.getTransaction().begin(); 

    A parentActivuty = new A(); 
    A subActivity1  = new A(); 
    A subActivity2 = new A(); 

    son.setParentActivity(parent); 
    daughter.setParentActivity(parent); 
    parent.setSubActivity(Arrays.asList(subActivity1, subActivity2)); 

    em.persist(parent); 
    em.persist(son); 
    em.persist(daughter); 

    em.getTransaction().commit(); 
} 

但在這種情況下，我在數據庫中有父對象，並希望堅持子對象什麼可能是可能的解決方案...？

Answer 1

你從數據庫中的父母，做的附件，並持續了兩個孩子：

A parent = em.get(A.class, parentId); 
A son = new A(); 
A daughter = new A(); 
son.setParentActivity(parent); 
daughter.setParentActivity(parent); 
em.persist(son); 
em.persist(daughter); 
parent.getSubActivities().add(son); 
parent.getSubActivities().add(daughter);