favorite pattern for adding functionality to an AR?

Do you have a preferred programming pattern for adding functionality to
an ActiveRecord? As an example, say I have some gnarly trig functions
for distance and bearing between pairs of latitude and longitude:

=== file: latlng.rb
module LatLng

def haversine_distance(lat1, lng1, lat2, lng2)
   ...
end

def bearing(lat1, lng1, lat2, lng2)
  ...
end
=== EOF

... and I want to mix in haversine_distance(other_ar) and
bearing(other_ar) methods into an ActiveRecord. The shim functions
would look like this: