Why does this Hibernate Criteria Query fails with "No explicit selection and an implicit one cold not be determined"?

I have an entity called Bucket, and I'm trying to build a criteria query to determine whether there is a Bucket stored with the "Name" property equals to "Bucket_1". So basically it is an exists query.

There is nothing special about the Bucket class, simpler impossible:

@Entity(name="Bucket")
@Table(name = "BUCKETS")
public class Bucket implements Serializable {
    private static final long serialVersionUID = 1L;

    @Column(name = "BUCKET_NAME", length=200)
    private String Name;

        ...
}

For the query, this is what I go so far:

CriteriaBuilder criteriaBuilder = em.getCriteriaBuilder();
CriteriaQuery<Boolean> superQuery = criteriaBuilder.createQuery(Boolean.class);

Class<? extends T> scopeClass = Bucket.class;
Root<? extends T> root = superQuery.from(scopeClass);

Path<?> attributePath = root.get("Name");
Predicate pred = criteriaBuilder.equal(attributePath, criteriaBuilder.literal("Bucket_1"));

Subquery<? extends T> subQuery = superQuery.subquery(scopeClass);
subQuery.where(pred);
Predicate where = criteriaBuilder.exists(subQuery);

superQuery = superQuery.select(where);

/* This line fails!! */
TypedQuery<Boolean> typedQuery = em.createQuery(superQuery);

boolean result = typedQuery.getSingleResult();

When I execute the query about, I get the following exception on the last line:

Caused by: java.lang.IllegalStateException: No explicit selection and an implicit one cold not be determined
    at org.hibernate.ejb.criteria.QueryStructure.locateImplicitSelection(QueryStructure.java:296)
    at org.hibernate.ejb.criteria.QueryStructure.render(QueryStructure.java:249)
    at org.hibernate.ejb.criteria.CriteriaSubqueryImpl.render(CriteriaSubqueryImpl.java:281)
    at org.hibernate.ejb.criteria.predicate.ExistsPredicate.render(ExistsPredicate.java:57)
    at org.hibernate.ejb.criteria.predicate.ExistsPredicate.renderProjection(ExistsPredicate.java:62)
    at org.hibernate.ejb.criteria.QueryStructure.render(QueryStructure.java:252)
    at org.hibernate.ejb.criteria.CriteriaQueryImpl.render(CriteriaQueryImpl.java:340)
    at org.hibernate.ejb.criteria.CriteriaQueryCompiler.compile(CriteriaQueryCompiler.java:223)
    at org.hibernate.ejb.AbstractEntityManagerImpl.createQuery(AbstractEntityManagerImpl.java:441)
    at com.specktro.orchid.io.connection.database.dao.internal.DefaultDAO.has(DefaultDAO.java:426)
    ... 28 more

I researched a lot but couldn't find anyone with the same error having it explained and/or fixed.

I also tried this way:

CriteriaBuilder criteriaBuilder = em.getCriteriaBuilder();
CriteriaQuery<Boolean> superQuery = criteriaBuilder.createQuery(Boolean.class);

Class<? extends T> scopeClass = Bucket.class;
superQuery.from(scopeClass);
Subquery<? extends T> subQuery = superQuery.subquery(scopeClass);
Root<? extends T> root = subQuery.from(scopeClass);

Path<?> attributePath = root.get("Name");
Predicate pred = criteriaBuilder.equal(attributePath, criteriaBuilder.literal("Bucket_1"));

Subquery<? extends T> subQuery = superQuery.subquery(scopeClass);
subQuery.where(pred);
Predicate where = criteriaBuilder.exists(subQuery);

superQuery = superQuery.select(where);

/* This line fails!! */
TypedQuery<Boolean> typedQuery = em.createQuery(superQuery);
boolean result = typedQuery.getSingleResult();

But I get the same exact exception.

Does anyone know why I get this and how to fix this query?

Thank you!! Eduardo

UPDATE:

I have been able to construct a query using exists the following way:

CriteriaBuilder criteriaBuilder = em.getCriteriaBuilder();

CriteriaQuery<Bucket> query = criteriaBuilder.createQuery(Bucket.class);
Root<Bucket> root = query.from(Bucket.class);
query.select(root);

Subquery<Bucket> subquery = query.subquery(Bucket.class);
Root<Bucket> subRootEntity = subquery.from(Bucket.class);
subquery.select(subRootEntity);

P开发者_如何学运维ath<?> attributePath = subRootEntity.get("Name");
Predicate pred = criteriaBuilder.equal(attributePath, criteriaBuilder.literal("Bucket_1"));
subquery.where(pred);
query.where(criteriaBuilder.exists(subquery));

TypedQuery<Bucket> typedQuery = em.createQuery(query);

boolean entityExists = typedQuery.getResultList().size() == 1;

Which resulted in a weird SQL generated like this:

select generatedAlias0 from com.test.Bucket as generatedAlias0 where exists (select generatedAlias1 from com.test.Bucket as generatedAlias1 where generatedAlias1.Name=:param0)

I guess the hard part is to get that select 1 ... going, where the result of the external query is Boolean and not the matching entity.

I know this one I got works, I'm now just trying to learn how to do it "properly", I believe there is a way...

---

Can you just do this?

boolean result = (session.createCriteria(Bucket.class)
       .add(Restrictions.eq("Name","Bucket_1"))
       .setProjection(Projections.count("Name"))
       .uniqueResult() > 0);