Hibernate惰性提取-获取子对象

Question

我的理解是，默认情况下，Hibernate将所有关系类型的FetchType设置为lazy。

在我的例子中，我有一个双向的OneToMany-ManyToOne关系，如下所示：

public class Parent{

    @Id
    @Column(name = "parent_id")
    private long parentId;

    @OneToMany(mappedBy = "parent")
    @JsonIgnore
    private List<Child> children;

    public List<Child> getChildren()
    {
        return this.children;
    }
}

public class Child
{
    private String name;

    @ManyToOne(optional = false) //every child must have a parent
    @JoinColumn(name = "parent_id")
    private Parent parent;

    public Child(Parent parent, String name)
    {
        this.parent = parent;
        this.name = name;
    }
}

public class ChildService
{
    public List<Child> getChildren(long parentId)
    {
        Parent parent = getParentRepository().findOne(parentId);
        return parent.getChildren(); //This returns null
    }

    public Child getNamedChild(long parentId)
    {
        Parent parent = getParentRepository().findOne(parentId);
        //??????????????? 
        //Not sure how to get the children of this specific parent, 
        //which has a specific name.
    }
}

public interface ParentRepository extends JpaRepository<Parent, Long> {
}

另一个问题是我如何得到一个有特定名字的孩子？是否可以通过存储库进行操作？还是需要getChildren（parentId）并迭代直到找到以特定方式命名的那个？

编辑：在一些建议之后，我继续这样实现了我的ChildRepository：

public interface ChildRepository extends JpaRepository<Child, Long> {
    @Query("SELECT child FROM Parent parent JOIN parent.child AS child WHERE parent.parentId = :parentId")
    List<Child> getChildren(@Param("parentId") String parentId);
    @Query("SELECT child FROM Parent parent JOIN arent.child AS child WHERE parent.parentId = :parentId AND child.childId = :childId")
    Child getChildByName(@Param("childId") Long childId, @Param("parentId") String parentId);
}

Answer 1

可以在ChildService类中注入EntityManager吗？我无法完全验证下面的代码，但您可以尝试以下操作

public class ChildService
{
    private EntityManager em;

    public List<Child> getChildren(long parentId)
    {
        Query query = em.createQuery("SELECT c FROM Parent p JOIN FETCH p.children AS c WHERE p.parentId = :parentId");
        query.setParameter("parentId", parentId);
        return (List<Child>) q.getResultList();
    }

    public Child getNamedChild(long parentId, String name)
    {
        Query query = em.createQuery("SELECT c FROM Parent p JOIN FETCH p.children AS c WHERE p.parentId = :parentId AND c.name = :name");
        query.setParameter("parentId", parentId);
        query.setParameter("name", name);
        return (Child) q.getSingleResult();
    }
}