Using acts_as_list, acts_as_tree, and STI in one model

Check it out y'all:
http://rafb.net/p/NJg5U147.html
(pastie was down)

I'm trying to make a tree which belongs_to another model, where you can
have sections or items as children, and sections can have sections or
items as well. Moreover, any siblings should be sortable.

For instance:

Root
- Section 1
- - Item A
- - Item B
- Item C
- Section 2
- - Item D
- - Section 3
- - - Item E
- - Item F

etc.

Those items are going to be "has_one" with another model.

Right now I'm getting this error: NameError: undefined local variable or
method `parent_id' but more generally, is there anything wrong with my
approach?