Hibernate - 加入不相关的对象

Question

我有一个要求，其中我必须使用 Hibernate HQL 连接两个不相关的对象。
这是示例 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，以便我可以拥有 Product 和 Item 的单独实例，而不是组合在另一个实例中。
有没有办法在 Hibernate 中做到这一点？

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，并且不需要对性能产生影响，因为 hibernate 具有延迟加载代理。

但当然，您可以在映射文件中加入未映射为关联的事物。引用 hibernate reference manual :

Multiple classes can appear, resulting in a cartesian product or "cross" join.

from Formula, Parameter

from Formula as form, Parameter as param

and

Queries can return multiple objects and/or properties as an array of type Object[]:

select mother, offspr, mate.name
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offspr

Or as a List:

select new list(mother, offspr, mate.name)
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offspr

Or - assuming that the class Family has an appropriate constructor - as an actual typesafe Java object:

select new Family(mother, mate, offspr)
from DomesticCat as mother
    join mother.mate as mate
    left join mother.kittens as offspr

You can assign aliases to selected expressions using as:

select max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n
from Cat cat

This is most useful when used together with select new map:

select new map( max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n )
from Cat cat

This query returns a Map from aliases to selected values.

结合一个简单的 where 子句，我们得到:

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

编辑 :我错过了你想要一个外连接。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