gistfile1.txt

in application.rb


    config.active_record.observers = :foo_observer

foo_observer.rb:

class FooObserver < ActiveRecord::Observer
  def before_update(foo)
  end
end

foo.rb model:

     class Foo < ActiveRecord::Base
        include SomethingFromPlugin
     end

This causes a 'uninitialized constant' for SomethingFromPlugin because the plugins aren't loaded yet.

rails/activerecord/lib/active_record/observer.rb:91 is causing the Foo model to load, through a chain of ActiveModel#observing => ActiveModel#observed_classes => ActiveModel#observed_class => constantize