我有一个名为Bucket的实体,我试图建立一个标准查询,以确定是否有一个Bucket存储与"Name"属性等于"Bucket_1"。这是一个exists查询
Bucket类没有什么特别之处,不可能更简单:
@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;
...
}
对于查询,这是我目前所做的:
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();
当我执行关于的查询时,我在最后一行得到以下异常:
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
我研究了很多,但没有找到任何有同样错误的人解释和/或修复。
我也试过这样做:
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();
但是我得到了完全相同的异常
有人知道为什么我得到这个和如何修复这个查询吗?
谢谢! !爱德华多
更新:
我已经能够用下面的方法构造一个查询:
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);
Path<?> 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;
这导致了一个奇怪的SQL生成如下:
select generatedAlias0 from com.test.Bucket as generatedAlias0 where exists (select generatedAlias1 from com.test.Bucket as generatedAlias1 where generatedAlias1.Name=:param0)
我想困难的部分是得到select 1…外部查询的结果是布尔值,而不是匹配的实体。
我知道这个我得到的作品,我现在只是试图学习如何做它"正确",我相信有一个方法…
你就不能这么做吗?
boolean result = (session.createCriteria(Bucket.class)
.add(Restrictions.eq("Name","Bucket_1"))
.setProjection(Projections.count("Name"))
.uniqueResult() > 0);