Question

我正在尝试使用以下类的JPA2条件API进行简单查询：

// a lot of imports

@Entity
public class Thing {
    enum Type { FIRST, SECOND, THIRD };

    @SequenceGenerator(name = "Thing_SeqGen", sequenceName = "Thing_Id_Seq", initialValue = 1000)
    @Id
    @GeneratedValue(generator = "Thing_SeqGen")
    private int id;

    private String name = "name";

    @Enumerated(EnumType.STRING)
    @ElementCollection(targetClass = Thing.Type.class)
    @CollectionTable(name = "TYPES", joinColumns = { @JoinColumn(referencedColumnName = "ID", name = "TYPE_ID") })

    private Set<Thing.Type> typeSet = new HashSet<Thing.Type>();
    public static void main(final String[] args) {
        new Thing().start();
    }

    public void start() {
        final Thing firstThing = new Thing();
        firstThing.setName("First one");
        firstThing.setTypeSet(EnumSet.of(Thing.Type.FIRST));
        final Thing firstAndSecondThing = new Thing();
        firstAndSecondThing.setName("Test2");
        firstAndSecondThing.setTypeSet(EnumSet.of(Thing.Type.FIRST, Thing.Type.SECOND));
        final Thing bareThing = new Thing();
        bareThing.setName("Test3");

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

        em.getTransaction().begin();
        em.persist(firstThing);
        em.persist(firstAndSecondThing);
        em.persist(bareThing);
        em.getTransaction().commit();

        em.getTransaction().begin();
        final CriteriaBuilder cb = em.getCriteriaBuilder();
        final CriteriaQuery<Thing> c = cb.createQuery(Thing.class);
        final Root<Thing> root = c.from(Thing.class);
        final Join<Thing, Set<Thing.Type>> typeJoin = root.join("typeSet");

        c.select(root).distinct(true).where(cb.isEmpty(typeJoin));

        final List<Thing> results = em.createQuery(c).getResultList();

        em.getTransaction().commit();
    }

    // getter/setter methods omitted
}

我想查询的内容：查找所有没有排版的内容。

完成这项工作的JPQL是：

select t from Thing t where t.typeSet is empty

JPQL查询返回一个预期的结果。条件查询不返回任何结果。 CriteriaBuilder已创建：

SELECT DISTINCT t0.ID, t0.NAME FROM THING t0, TYPES t1 WHERE (((SELECT COUNT(t2.ID) FROM THING t2 WHERE (t1.TYPE_ID = t0.ID)) = 0) **AND (t1.TYPE_ID = t0.ID)**)

最后一次theta-join（标记为**）将全部杀死。我不知道为什么表格THING被指定两次(THING to, THING t1)。

显然我做错了。但我不知道是什么错。

Answer 1

我猜测问题是你试图在Criteria案例中进行显式连接，而在JPQL中则没有。因此省略连接并执行类似

的操作

Metamodel model = emf.getMetamodel();
ManagedType thingType = model.managedType(Thing.class);
CollectionAttribute typeSetAttr = thingType.getCollection("typeSet");
c.select(root).distinct(true).where(cb.isEmpty(root.get(typeSetAttr)));

然后，这应该转换为与您发布的相同的JPQL ...或者至少它适用于DataNucleus JPA实现。

关于ElementCollection与JPQL方法的CriteriaBuilder.isEmpty

1 个答案: