gem 'constance'

bundle install

...

# Debug class/module loading without stacks
Constance.configure do
  self.debug = true
  self.verbose = false
end

module MyRailsApp
  class Application < Rails::Application
  ...

Constance.debug = true

Constance.verbose = true

Constance.proc = lambda{|from_mod, const_name| puts "my lambda was called from_mod=#{from_mod.inspect} const_name=#{const_name.inspect} caller: #{caller.inspect} and I can return any class or constant I want, or call some other class or method. But, for kicks I'll return nil"; nil}

Constance.caller_search_mapping = {/(\/app\/|\/lib\/)/ => {'Journey' => 'Fantastic::Journey'}} # go through the caller stack to see if was called from a local or gem file that had app or lib in the path

Constance.configure do
  self.debug = true
  self.verbose = true
  self.proc = lambda{|from_mod, const_name| puts "my lambda was called from_mod=#{from_mod.inspect} const_name=#{const_name.inspect} caller: #{caller.inspect} and I can return any class or constant I want, or call some other class or method. But, for kicks I'll return nil"; nil}
  self.caller_search_mapping = {/(\/app\/|\/lib\/)/ => {'Journey' => 'Fantastic::Journey'}} # go through the caller stack to see if was called from a local or gem file that had app or lib in the path
end

Constance::Resolver.will_replace(Journey)

Loaded missing constant from_module: Object const_name: :Foo using ActiveSupport::Dependencies. Got: Foo
Loaded missing constant from_module: Object const_name: :ApplicationController using ActiveSupport::Dependencies. Got: ApplicationController
Loaded missing constant from_module: InheritedResources const_name: :Base using ActiveSupport::Dependencies. Got: InheritedResources::Base
Loaded missing constant from_module: Object const_name: :Bar using ActiveSupport::Dependencies. Got: Bar
Loaded missing constant from_module: Object const_name: "Foobar" using ActiveSupport::Dependencies. Got: Foobar