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.

Max