Two ways that take a case statement out of your controller.

example_a.rb

class UserConroller
  def index
    @users = User.sort_by(params[:sort_by])
  end
end

class User
  def sort_by(sort_by)
    case sort_by
      :created_at order(:created_at)
      :last_updated order(:last_updated)
      else order(:id)
    end
  end
end

example_b.rb

class UserController
  def index
    @users = user.order(params[:sort_by] || :id)
  end
end

I am maybe misremembering an example from Jeff Casimir's talk at RailsConf 2011 about moving complexity out of Controllers and into Models (among other, related topics). He presented example_a (as best I can remember) as a better alternative to putting a case statement in your action (which it is). I wondered why the solution wasn't example_b? Maybe because example_a was simplified for example and a real case might be more complex (and have more lines of code in each case option), but maybe example_b is poor for some reason I don't know. Please leave thoughts in the comments.

Well, example B isn't precisely equivalent to A. B allows the end user to sort by anything they like. A allows only created_at, last_updated, or id. (For example: foo.com/users/index?sort_by=nose_length)

Also: is this a Ruby 1.9 thing? Where are the "when/then" clauses in the case statement? If they are optional I didn't know it. :)

Yeah. I may have fucked up the case syntax. I never use it and was too lazy to look it up. Hopefully it's still clear what the heck I'm blathering about. Your exclusivity point is well made.