I'm trying to connect a parent-child dimension to my fact table.
I would like to browse cube measure (sold items quantity) through the
dimension "Groups" representing items family (tree structure).

Here are the tables:

Fact table (items sold with quantity):
------------------
IdItem;Quantity
1;5
2;2
3;1
1;6
3;8
------------------

Dimension table (parent-child tree representing groups item):
------------------
IdGroup;GroupName;IdParent;IdItem
1;AAA;-1;-1
2;BBB;1;-1
3;CCC;1;-1
4;O1;2;1
5;O2;2;2
6;O3;3;3
------------------

If I connect dimension to fact table with a regular relation (on IdItem
attribute of each table) and browse cube, I obtain for each node the
same quantity (22)

AAA  (22)
-BBB (22)
--O1 (22)
--O2 (22)
-CCC (22)
--O3 (22)

I would like to obtain this result:

AAA  (22)
-BBB (13)
--O1 (11)
--O2 (2)
-CCC (9)
--O3 (9)

If I use a "reference relation" with a middle table it's ok...
but I don't understand the difference!!!!!!

Can someone help me?
thanks