List of List

GrønnDemon · **Joined:** Thu Dec 01, 2005 1:09 pm **Posts:** 33

Hi,

I have many objects which can serve as parents. But actually, only very few of them do.

I have a child table with foreign keys to their parents, a common many-to-one relation.

In my program I want to print all a parent's children in one row, so I'd like to either have an array of all parent objects which actually do have children, or a list of lists of children, where each children list contains all the children of one parent.

In code this would either be:

Code:

Parent[] parents; //only the parents that actually have children

or:

Code:

Vector<Vector<Child>> childrenPerParent; //outer vector contains one list per parent

or:

Code:

Child[][] children;

Any suggestions will be highly appreciated.

-Marvin

kochcp · **Posted:** Mon May 01, 2006 9:45 am

well, if you're using hibernate, it seems like the first choice would make more sense. And each parent object has a list of its children.

GrønnDemon · **Joined:** Thu Dec 01, 2005 1:09 pm **Posts:** 33

Of course. But I don't want to load ALL parent objects as many of them don't have any children at all.
I'm only interested in the ones with children.

I've tried this approach (from the parent class):

Code:

<set name="Children" inverse="true" cascade="delete">
    <key column="PARENT_ID" />
    <one-to-many class="Parent" />
</set>

And then I tried the followin HQL statement:

Code:

from Parent p where p.Children is not null

Unfortunately I get this error message:

Quote:

unindexed collection before [] [from Parent p where p.Children is not null]

Any idea?

GrønnDemon · **Joined:** Thu Dec 01, 2005 1:09 pm **Posts:** 33

Oh, and I use NHibernate, but I don't think that makes any difference...

kochcp · **Posted:** Mon May 01, 2006 1:55 pm

can't you just do a select all on the table with the "children" and order by the foreign key

GrønnDemon · **Joined:** Thu Dec 01, 2005 1:09 pm **Posts:** 33

Yeah, got it to work now using "select distinct c.Parent from Child c".