best way to add a method to the Array class

Ruby classes are open, so you can simply do:

class Array   def sum      inject(0) { |sum,v| sum+=v }   end end

You can also redefine existing methods, but you'll have to be very careful with that. alias is your friend.

Cheers, Max

where would that code snippet go?

In the application controller?

No. Create a new file under lib, for example array_extensions.rb and you should be fine.