See Rails ticket: http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/1286

A Model that depends on an application initializer, and that has an Observer, fails to load when a Gem dependency is missing.

In this example app we have a Model Splat that calls the class method "extend_me" which is defined on ActiveRecord::Base by the application initializer in initialize_splat.rb.

If the required Gem (arbitrary gem "Bill-route_name_for" is specified in the example) is missing (not yet installed) then rake gems:install fails before gem installation is attempted to wit:

$ rake gems:install
(in /Users/Bill/Projects/Rails/ObserversLoadedWhenGemsAreMissing)
rake aborted!
undefined local variable or method `extend_me' for Splat(Table doesn't exist):Class

This is because the application initializer in initialize_splat.rb needs to be loaded
before the model in splat.rb is loaded. But that application initializer is skipped
when gem dependencies are missing.

The model (Splat) has an observer in models/splat_observer.rb
Rails::Initializer#prepare_dispatcher does not respect the @gem_dependencies_loaded
flag so it calls Dispatcher.define_dispatcher_callbacks which in turn calls
ActiveRecord::Base.instantiate_observers.

side note: Rails::Initializer#load_observers does check @gem_dependencies_loaded and it won't load observers when gems aren't loaded (good!)

Recommend modifying Rails::Initializer#prepare_dispatcher's guard condition

is:

def prepare_dispatcher
  return unless configuration.frameworks.include?(:action_controller)
  …
end

should be:

def prepare_dispatcher
  return unless configuration.frameworks.include?(:action_controller) && @gem_dependencies_loaded
  …
end

Since we don't need the dispatcher if Gem dependencies are missing.