java - Hibernate Criteria with Projections.groupProperty 无法返回完整的 hibernate 对象 (ClassCastException)

Question

我是 Hibernate 的新手，在我的 hibernate 类上添加“distinct”限制时遇到了问题。

@Entity
public class TaggedOffer {
   private Long tagged_offers_id;
   private String brand;
   private Long cid;
   private Date created_date;
   //Getter and Setter and more fields
}

之前，我们创建了一个 hibernate 查询，如下所示:

public DetachedCriteria build(final TaggedOfferRequest request) {

        DetachedCriteria criteria = DetachedCriteria.forClass(TaggedOffer.class);

        criteria.add(Restrictions.eq("brand", request.getBrand()));
        criteria.add(Restrictions.in("cid", request.getCids()));

        // sort by date
        criteria.addOrder(Property.forName("createdDate").desc());

        return criteria;
}

这将创建以下(有效的)SQL 查询:

select
        this_.tagged_offers_id as tagged1_2_3_,
        this_.brand as brand2_3_,
        this_.cid as cid2_3_,
        this_.created_date as created6_2_3_
    from
        site.tagged_offers this_ 
    where
        this_.brand=? 
        and this_.country_code=? 
        and this_.cid in (
            ?, ?
        ) 
    order by
        this_.created_date desc limit ?

棘手的部分来了。我们现在需要确保返回的结果在 cid 字段上是不同的。意思是，返回尽可能多的结果，前提是每条记录都有一个与之关联的唯一 cid。

我在 SQL 中对此进行了研究，看起来最简单的方法就是在查询中使用 group by cid。就 hibernate 标准而言，这基本上是我一直在尝试的:

public DetachedCriteria build(final TaggedOfferRequest request) {

        DetachedCriteria criteria = DetachedCriteria.forClass(TaggedOffer.class);

        criteria.add(Restrictions.eq("brand", request.getBrand()));
        criteria.add(Restrictions.in("cid", request.getCids()));

        // sort by date
        criteria.addOrder(Property.forName("createdDate").desc());

        // ** new ** distinct criteria
        criteria.setProjection(Projections.groupProperty("cid"));

        return criteria;
}

这几乎创建了我正在寻找的 SQL，但它后来抛出了一个类转换异常(因为它只是选择了 cid 字段而不是整个对象)。

select
    this_.cid as y0_ 
from
    site.tagged_offers this_ 
where
    this_.brand=? 
    and this_.country_code=? 
    and this_.cid in (
        ?, ?
    ) 
    and tagtype1_.tag_type=? 
group by
    this_.cid 
order by
    this_.created_date desc limit ?

异常(exception)情况:

java.lang.ClassCastException: java.lang.Long cannot be cast to com.mycompany.site.taggedoffers.dao.model.TaggedOffer

知道如何使用投影来做我想做的事吗？

感谢您的帮助。

Answer 1

为您需要的所有列添加投影。

    ProjectionList projectionList = Projections.projectionList();
    projectionList.add(Projections.groupProperty("cid"));
    projectionList.add(Projections.property("tagged_offers_id"));
    ...
    criteria.setProjection(projectionList);