Last Updated: 2024-04-23

How does reloading work (e.g. in Rails, pre Zeitwerk):

• Before each request (or console command), Rails calls Object#remove_const for every constant - e.g. for User, it calls Object.send :remove_const, "User"

• Because of this, any caching/storage of a reloaded constant in a place that isn't reloaded (e.g. as a global variable) will cause bugs:

  # config/initializers/configure_payment_gateway.rb
  # careful: global variable being set
  $PAYMENT_GATEWAY = Rails.env.production? ? RealGateway : MockedGateway

Now, even when MockedGateway gets changed, $PAYMENT_GATEWAY is stuck with the original version when the initializer ran. I.e reloading does not work.

References

• https://guides.rubyonrails.org/autoloading_and_reloading_constants.html#reloading-and-stale-objects