This is a new issue for me that I've never seen before. After a little
reading it looks as though I have an object in the session that is
derived(?)that Rails cannot figure out? Is the problem that I have all
the class definitions in the same model file i.e. in top_class.rb

So in essence I have:

class TopClass < ActiveRecord::Base
acts_as_tree
:
:
end

class AnotherClass < Tree
:
:
end

class YetAnotherClass < Tree
:
:
end

Any help would be greatly appreciated.