gistfile1.rb

# Goal: put a non-Rails-aware Ruby library using normal `require`s in
# lib/sim. Have it transparently reloaded between requests like Rails app
# code is.
#
# The code here goes inside of your configure block in
# config/environments/development.rb. There are two parts, commented inline.

# Reload code whenever the simulator changes.
config.watchable_dirs["lib/sim"] = [:rb]
config.watchable_files << "lib/sim.rb"

# Manually unload and reload the simulator before every request. This assumes
# that there's a lib/sim.rb loading files from lib/sim/.
ActionDispatch::Reloader.to_prepare do
  # If the library's top level module is currently loaded, unload it
  if Object.const_defined?(:Sim)
    Object.send(:remove_const, :Sim)
  end

  # Instruct ruby to "unrequire" all of the simulator's files.
  # CAREFUL: make sure this only matches your library's files.
  sim_base = File.expand_path(File.join(File.dirname(__FILE__), '../../lib/sim'))
  $".delete_if do |s|
    is_in_sim_dir = s.start_with?(sim_base + "/")
    is_sim_file = s == sim_base + ".rb"
    is_in_sim_dir || is_sim_file
  end

  # Re-require your library
  # Note: because we removed all files previously required they will be reloaded
  # even if you didn't use load/autoload in your library.
  require_relative "../../lib/sim"
end

I think @BrokenLadder is saying that objects which inherit from your lib might not act as expected.