that's not an implementation of nested set trees
Sorry, I missed the 'nested set' portion of the request. What I supplied was an adjacency list implementation (according to the link you sent ... thanks BTW).
But then I don't see how you're even getting the children for the "each" in nested_display.
The acts_as_tree method defines a bunch of methods in your model object, including children, siblings, self_and_siblings, and ancestors. It also defines a roots method in the class object.
Theoretically, you would just create a acts_as_nested_set method that could create all the same methods, but using the nested set implementation. I'll have to review your linked article in more detail before I could say how easy that might be.