> ... but when I visit the web page in question I get "No method
> 'my_compress' for Array ..."
> I'm probably doing something fundamentally wrong - what could/should I
> be doing?
You should be putting your model logic in the model, not in the
controller.
on top of that, by putting it inside the controller like that you've
created a new module called LookupController::Enumerable rather than
extending Enumerable. If I were you I'd keep extensions to core
classes somewhere in lib.