Hi,

I was wondering if it is possible to restrict a OneToMany relationship by a property on the destination table?

i.e. instead of having a single relationship that I have to cycle through to pull out the values that I want, I would like to introduce 2 OneToMany relationships, one for each type of the destination class,

so instead of
getAllChildren()

I would like to have:

getChildrenOfType1()
getChildrenOfType2()

Where I know the exact number of types that there are, so this is not an issue. For now I will write some code to cycle through the children and generate the 2 different lists that I require, but it would be nice if Hibernate could do this for me through some annotation,

Thanks,
Osian

It is possible to add a @Where annotation (or where attribute if you use xml mapping files) that is added as part of the SQL to fetch the collection. See http://docs.jboss.org/hibernate/stable/ ... on-enhance and http://docs.jboss.org/hibernate/stable/ ... ns-mapping

Ah, perfect, thanks, I thought it would be possible in some way.

Unfortunately, this doesn't quite meet what I want, because it doesn't use the names of the bean properties, it requires the SQL column name, and due to this, I am also concerned that it will become database specific with some of the values being passed to the query, i.e. Oracle & Hibernate treat booleans differently.

Also, I require that the returing set is restricted on a value from an association table, and not directly from the table within the SortedSet.

Any ideas? In the meantime I am going to have to do this manually, but it would be great to see Hibernate do this for me,

Thanks,
Osian

Hi Osian

I can see 2 possible answers for your question.

Answer 1
--------
Try to use filters.
Well, this solution is not based on annotation, but it is a regular Hibernate functionality. And, using filter, you can operate using property name, not database column name.
See: http://docs.jboss.org/hibernate/stable/ ... -filtering

So, in this technique, you will still get getAllChildren() Hibernate mapping, but using filters, you will never load this collection into memory. So you can have millions of children per particular entity, but using filters, Hibernate will generate database hit, every time, you use filter.

Answer 2
--------
Did you try to map your Children types as separated children type using “Inheritance mapping”?
http://docs.jboss.org/hibernate/stable/ ... nheritance

I’ve got impression that you could consider map your children as different type, using “discriminator” Hibernate technique.
Well... Then, you can try to change your mapping and give a try to the following mapping:



I don’t know if it works. I didn’t try this trick.

Regards, Adam Woźniak

The @WhereJoinTable annotation is used to filter on the association table. Both this and @Where use SQL fragments, so they are sensitive to database dialects.

There is a method in the Session interface that help with filtering collections: Session.createQuery(Object collection, String filter). It returns a Query object. I don't know if that is useful or not for you. The example in the original post is probably better to map the allChildren collection and then split it up when getChildrenOfType1() or getChildrenOfType2() is called. It mean that only one query is needed instead of two.

An exception would be if one of the sub-collections is large but seldom accessed and the other is small but accessed very often. With large I mean several thousands of records. In this case it is probably quicker to only load the sub-collection that is needed and avoid loading lots of objects that are usually not needed.

I've just used local transient Lists for now due to time constraints, but I believe that the typing way would be the way to go, so hopefully I will get time to do this.

Thanks everyone for your replies.