Need some assistance with routing -- 'resource' using natural key

I want to create a resource, let's say 'cars', which are accessed by license_number rather than id. The column in question is guaranteed to be unique and unchanging (and no, it's not actually a car table).

Best way to go about this?

Thank you, Isak

class Cars < ActiveRecord::Base   ...   def to_param     return self.license_number   end   ... end