Sbastien M. wrote in post #1034503:
> Quick question though
>
> Why don(t u need the 'respond_to?' method
> We still have to check if object has a price method in order to return
> nil
> if not. Or is there something i missed?

You need respond_to? if you may pass in a non-Item instance to be added to an Item (or rather, as this is Ruby, an instance which does not respond to the 'price' method).  Myself, I would try not to do that, but that depends on the design of the rest of your program.