[AccessD] Treeview Problems

Some of you may have read my stuff about data-driven treeviews. Lately I
have run into a problem using my own code, and am looking for a graceful
way out.

If you've read or at least run the code, then you know that it reduces
the population of any given level to a simple select statement, using a
query, a table, a view or a UDF. 

However, my current problem is this:

Let's suppose that you have parent table P and child table C, which
references an FK F (i.e. OrderDetailType.... Assuming that there can be
several types of OrderDetail, say CarRental, CampGroundSite,
CanoeRental, etc. ((hey, it's summer and I need to revisit Killarney
Park, one of the most beautiful places in the world, so forgive my
examples)).

Assume that Treeview level 1 is Customer. TV level 2 is Orders. Treeview
Level 3 is all the possible order detail types. TV 4 is the detail types
arranged under TV3.

I.e.

1:
ABC Company
DEF Company

2:
Order 123 from ABC
Order 125 from ABC
Order 126 from DEF
Order 139 from DEF

3:
CarRental
CampGroundSite
CanoeRental
Etc.

4:
Specific instances of CarRental, CampGroundSite, CanoeRental, etc.

My code so far can handle levels 1 and 2 easily, one statement per
level.

The problem is at level 3. I want to populate the treeview at level 3
with all the order detail types (CarRental, CampGroundSite, CanoeRental,
etc.) for each Order. It's a simple UNION query to do this, but where do
I get the unique key?

Generalizing my problem, I have a simple (and dare I say it pretty cool)
way of populating the data-driven levels of a treeview. But that's
insufficient for my current problem. I need to populate the
OrderDetailTypes level with all possible order detail types and then
sub-populate level 4 with the corresponding children. I'm at a loss how
to achieve this.

One way to do it, which would fall conveniently into my current scheme,
is to populate a bridge table (OrderDetailTypes) with one of each
OrderDetailType... But that sucks, IMO. It stores a bunch of data that
is most of the time unnecessary. I HATE unnecessary detail! I'll go
there if I have to, but it makes me gag.

Generalizing the problem....

Given: code that can populate any tree-level with a simple select
How to insert a level that is not data-driven or alternatively is
data-driven but not with related FKs
Treeview nodes insist upon unique keys (understandably so). But how
should I manufacture said unique keys given that no physical rows
correspond to them.
Maybe the generalization is: how to relate a grandchild to a parent in
the absence of parental data? That might be an incorrect phrasing, but
that's what I mean.

I hope that I have stated the problem clearly enough for you to follow
the logic. If not, please reply for further clarification.

Any suggestions much appreciated.

TIA,
Arthur