how to override enumerable compare for attributes?

How would I override <=> for certain attributes of a class?

Say I have the following:

class Workout < ActiveRecord::Base
# columns in db: time, weight, reps, distance
end

I want to be able to sort by results, to list workouts by best time,
weight, etc. But the ordering is different, because a low time is
better than a high time, while a high weight is better than a low
weight, etc.

Right now I'm doing a class method with an if statement like this:

def self.ranked_by(compare)
  if compare == :time
    find(:all).sort_by{|workout| workout.time}.reverse
  else
    find(:all).sort_by{|workout| workout.send(compare)}
  end
end

That works, but it's ugly and the ugliness tends to replicate itself.
I could simplify my code by overriding the <=> method for the time
attribute so that I could simply do:

def self.ranked_by(compare)
  find(:all).sort_by{|workout| workout.send(compare)}
end

But how to do that?