You can clone with
# what? why does your production code care what environment it is in? SILLY!
unless Rails.development? || Rails.test?
# in code, no more environment check
# in config/development.rb
CreatesReservationTimer = Timers::Null
# in config/production.rb
CreatesReservationTimer = Timers::IronWorker
I like the way the "production" code ends up, but having the constant defined via initializer would likely surprise me. At least the first time I discovered it. And every time I forgot and re-discovered it. Probably worth it though.
...and the 'old_school_use' has the advantage of having it all together, and making reading easier.