在JPA中选择父类和所有子类的计数

Question

我有以下JPA实体结构。

@Entity
@Table(name = "PARENT_DETAILS")
class Parent{

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "parent_details_seq")
    @SequenceGenerator(name = "parent_details_seq", sequenceName = "PARENT_DETAILS_SEQ", allocationSize = 1)
    @Column(name = "PARENT_ID")
    private long parentId;

    @Column(name = "PARENT_NAME")
    private String parentName;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "childPK.parent", cascade = CascadeType.ALL)
    private Set<Child> child;
    //setters and getters
}

@Entity
@Table(name = "CHILD_DETAILS")
public class Child {

    private ChildPK childPK;

    public void setProgramProcessesPK(ChildPK childPK) {
        this.childPK = childPK;
    }

    @EmbeddedId
    public ChildPK getChildPK() {
        return childPK;
    }

  }


@Embeddable
public class ChildPK implements Serializable {
    private static final long serialVersionUID = 1L;
    private Parent parent;

    private long childId;

    @Column(name = "CHILDID")
    public long getChildId() {
        return childId;
    }

    public void setChildId(long childId) {
        this.childId = childId;
    }

    @ManyToOne
    @JoinColumn(name = "PARENT_ID", referencedColumnName = "PARENT_ID", nullable = false)
    public ParentDetails getParent() {
        return parent;
    }
}

我想写一个JPA查询，它将返回给定parent_id的PARENT_NAME和所有子级的计数。

t我能想到的唯一解决办法是加入并编写一个复杂的条件查询。

我想不出使用简单的JPA查询获得结果的方法。

有更简单的方法吗？

Answer 1

您可以使用以下JPA命名查询：

private static class ParentChildsNumber {
    public String parentName;
    public Integer childsNum;

    public ParentChildsNumber(String parentName, Integer childsNum) {
        this.parentName = parentName;
        this.childsNum = childsNum;
    }
}

@NamedQuery(name="getParentChildsNumberQuery", query="SELECT NEW ParentChildsNumber(p.parentName, SIZE(p.child)) FROM Parent p WHERE p.parentId = :parentId GROUP BY p.parentId, p.parentName")

通过以下方式在您的代码中使用它：

@PersistenceContext(unitName="YourPersistentUnit")
private EntityManager em;

em.createNamedQuery("getParentChildsNumberQuery", ParentChildsNumber.class).setParameter("parentId", parentId).getSingleResult();

Answer 2

你试过大小吗？像“从父父级中选择parent.parent名称、大小（parent.child）”这样的方法可能会奏效。