Question

我有一个要求，其中我必须使用Hibernate HQL加入两个不相关的对象。这是样本POJO类

class Product{
int product_id;        
String name;
String description;
}

和

Class Item{
int item_id;
String name;
String description;
int quantity;
int product_id; //Note that there is no composed product object.
}

现在我想执行一个类似的查询 select * from Product p left outer join Item i on p.product_id = i.item_id

我想要一个多维数组作为此查询的输出，这样我就可以拥有Product和Item的单独实例，而不是另一个实例。在Hibernate中有没有办法做到这一点？

Answer 1

通过存储关联对象的id而不是关联对象本身来表示对象之间的关联是不常见的。存储对象更具表现力和类型安全性，并且不需要对性能产生影响，因为hibernate具有延迟加载代理。

但是当然你可以在映射文件中加入未映射为关联的东西。引用hibernate reference manual：

可能会出现多个类在笛卡尔积中或“交叉” 加入。
from Formula, Parameter

from Formula as form, Parameter as param

and

查询可以返回多个对象和/或作为类型数组的属性对象[]：
select mother, offspr, mate.name
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offspr
或作为清单：
select new list(mother, offspr, mate.name)
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offspr
或 - 假设班级为家庭有一个合适的构造函数 - 作为实际的类型安全Java对象：
select new Family(mother, mate, offspr)
from DomesticCat as mother
    join mother.mate as mate
    left join mother.kittens as offspr
您可以为选定项指定别名表达式使用as：
select max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n
from Cat cat
这在一起使用时最有用选择新地图：
select new map( max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n )
from Cat cat
此查询从别名返回Map 选定的值。

将此与简单的where子句相结合，得到：

select product, item
from Product product, Item item
where item.product_id = product.product_id

修改：我错过了您想要外部加入。在这种情况下，我不知道除了映射关联之外的其他方法，因此您可以对其进行普通连接。请注意，这不需要特定形式的查询结果，即您仍然可以执行以下操作：

select product, item
from Item item
left join item.product as product

Answer 2

有一种解决方法可以在hql here中加入id。

但是，我建议在这里使用本机SQL查询。

Answer 3

您可以使用subselect

 from Product p where p.product_id in (select item_id from Item)

Answer 4

在select子句中尝试subselect：

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Hibernate - 加入un相关对象

4 个答案: