It creates the left and right values automatically (I'm the author of DBIx::Tree::NestedSet) when you invoke any method that deletes/moves/adds nodes. What use would the module be if it didn't? Did you even try the code I posted in the other thread you created?
With a nested set you don't need a parent ID: the hierarchy is created via left and right. Managing a parent id is redundant.
Traversing the tree is right in the docs.
-Any sufficiently advanced technology is
indistinguishable from doubletalk.