Can anyone help me with this design problem?

I've got a tree of nodes. Leaf nodes can have 0 or 1 workrecords.
Other nodes cannot have a work record pointing to them. My tables
look like this:

create table treenodes
(
idcode integer primary key,
parentnode integer,
...
foreign key (parentnode) references treenodes(idcode)
);

create table workrecords
(
idcode integer primary key,
...
foreign key (idcode) references treenodes(idcode)
);

The unique index on workrecords.idcode ensures only one workrecord
can be attached to a node. But now can I ensure workrecord's are not
assigned to non-leaf nodes???? Something like:

[trigger before insert or update on workrecords]:
select count(*) from treenodes where parentnode = new.idcode into :i;
if (i > 0) then
exception must_be_leaf;

...can only see committed records. What if something is pending???

This project is at v early stages, so table redesign is no problem
if something radical is needed.

I really want to be able to dirty-read (grin).

Hope someone can help me out!

All I can think of is having another table of 'leafnodeid's.
Touching the record when a workrecord is created for it, and
deleting it when a child node is assigned.

Thanks,

John