Form update help

Im trying to teach myself rails by creating a training log application
and Im wondering what the best practice for generating activities when
a certain sport is selected would be. I generated a athletes scaffold
and edited the form with a select box for the sport and once you selct
the sport I want the application to automatically generate activities
or workouts for that given sport. I initially did this by inserting
this code into the create and update methods of the controller:

  if @athlete.sport == "Cross Country Skiing"
    @athlete.activities = %w( SkateSki ClassicSki RSCL RSSK Gym)
    elsif @athlete.sport== "Road Biking"
    @athlete.activities = %w( RoadBike MountainBike Gym)
    end

However this causes a small anomaly when I update the info the first
time it sees the original sport when it calls the @athlete =
Athlete.new(params[:athlete]) from the database. This results in it
not changing the array of activities, making me have to repeat the
edit once more before it sees the new sport in the database entry.
There must be a better way of doing this, is it possible to put this
logic in the model before it saves/updates the object? Any
suggestions?

Thanks in advance!
Graeme