A few questions about ltree

[Alban Hertroys <alban(at)magproductions(dot)nl>]

Yesterday ltree was mentioned to be a good system for tree structured 
table data. I and a colleague of mine have been playing around with the 
examples and the (rather sparse) documentation, but we're stuck on a few 
questions...

How does one guarantee referential integrity using ltrees? It doesn't 
seem to do so by itself, but can it refer a parent node directly?

We assume you can do this:

CREATE TABLE my_tree (
	path ltree PRIMARY KEY,
	parent ltree REFERENCES my_tree(path)
);

In this case a tree would look something like:
parent | path
--------------------------
(NULL) | A
A | A.B
A.B | A.B.D
A | A.C

That's the "classical" way, which is also used in our current 
implementation with integers instead of ltrees, but it's not very easy 
to query efficiently (at least ordering seems to remain a problem).

Maybe something along the lines of the following is possible?:

CREATE TABLE my_tree (
	path ltree PRIMARY KEY REFERENCES my_tree(path)
);

Data would look like:
path
-----------------------
A
A.B
A.B.D
A.C

With A.B and A.C referencing A in their parent record and A.B.D 
referencing A.B

What I like about this solution is that only one ltree path per node is 
required, and that the root node doesn't need a parent reference. The 
question is whether this is/can-be-made possible...

Do ltrees know that a node with path 'A.B.D' references it's parent 
'A.B'? I mean, can ltree 'A.B' equal ltree 'A.B.D' somehow while the 
strings are unequal?
Can it be made to know that somehow (functional foreign keys or 
something - maybe using "ltree_isparent(ltree, ltree)")?

I can determine things like this with a few experiments, but I want to 
know "the right way" to work with ltrees and referential integrity. How 
do people use this?

--
Alban Hertroys
alban(at)magproductions(dot)nl

magproductions b.v.

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