加入fetch:"查询指定的连接提取,但获取的关联的所有者在选择列表中不存在"

Question

我有以下代码:

public class ValueDAO  implements BusinessObject<Long> {

    private Long id;
    private String code;
    private ClassDAO classDAO ;
        ....
}

public List<String> getCodesByCodeClass(Long classId) {
    String select = "select distinct val.code from ValueDAO val left " +
        "join fetch val.classDAO ";
    String where = "where val.classDAO.id = ? order by val.code";

    return getHibernateTemplate().find(select + where, classId);
}

它引发了一个例外:

 org.hibernate.QueryException: query specified join fetching, but the owner of the fetched association was not present in the select list

结果我想得到的只是代码.

Answer 1

join fetch val.classDAO.b意思是"在提取时val,也可以获取classDAO链接到val".但是您的查询无法获取val.它只取val.code.所以抓取没有任何意义.只需删除它,一切都会好起来的:

select distinct val.code from ValueDAO val 
left join val.classDAO classDAO
where classDAO.id = ? 
order by val.code

但有些说明:

• 进行左连接,然后添加一个限制,classDAO.id = ?意味着连接实际上是一个内连接(因为classDAO不能为null并且同时具有给定的ID)
• 命名你的实体XxxDAO非常混乱.DAO和实体根本不是一回事.

鉴于上述情况,可以将查询重写为

select distinct val.code from ValueDAO val 
where val.classDAO.id = ? 
order by val.code