Hibernate - join un related objects
I have a requirement, wherein I have to join two unrelated objects using Hibernate HQL. Here is the sample POJO class
class Product{
int product_id; 开发者_JAVA技巧
String name;
String description;
}
and
Class Item{
int item_id;
String name;
String description;
int quantity;
int product_id; //Note that there is no composed product object.
}
Now I want to perform a query like select * from Product p left outer join Item i on p.product_id = i.item_id
I want a multidimensional array as an output of this query so that I can have separate instances of Product and Item, instead of one composed in another. Is there any way to do this in Hibernate?
It is uncommon to represent an association between objects by storing the id of the associated object, rather than the associated object itself. Storing the object is more expressive and type safe, and need not have a performance impact since hibernate features lazy loading proxies.
But of course you can join things not mapped as associations in the mapping file. Quoting the 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.
Combining this with a simple where clause, we get:
select product, item
from Product product, Item item
where item.product_id = product.product_id
Edit: I missed that you want an outer join. In that case I don't know a way other than mapping the association so you can do an ordinary join on it. Note that this does not necessitate a particular form of the query result, i.e. you can still do:
select product, item
from Item item
left join item.product as product
There is a workaround to join over ids in hql here.
However, i recommend using a native sql query here.
You can use subselect
from Product p where p.product_id in (select item_id from Item)
Try subselect in select clause:
select p, (select i from Item where p.product_id = i.item_id) from Product p
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