support.rb

module RailsBestPractices
  module Spec
    module Support

      def within_observable_scope
        observer_class = self.description.split('::').inject(Object){|k,c| k.const_get(c) }
        has_applied_tweak, tweaks = apply_observer_tweaks(observer_class)
        begin
          yield(observer_class.instance)
        ensure
          unapply_observer_tweaks(observer_class, tweaks) if has_applied_tweak
        end
      end

      def apply_observer_tweaks(observer_class)
        observer, is_unwanted_observer_initialized = nil
        orig_update_meth = :_original_update_observee_xyz_ # some weird name that should never clash with others

        if ObjectSpace.each_object(observer_class){|o| observer = o }.zero?
          # This is the case when the observer is never meant to exist in this environment
          [true, {:method => orig_update_meth}]
        elsif observer.respond_to?(orig_update_meth)
          # This is the case when the observer is never meant to exist in this environment,
          # yet it has already been tampered with, and we want to undo tampering
          (class << observer_class.instance; self; end).class_eval do
            alias_method :update, orig_update_meth
          end
          [true, {:method => orig_update_meth}]
        end
      end

      def unapply_observer_tweaks(observer_class, tweaks)
        orig_update_meth  = tweaks[:method]
        (class << observer_class.instance; self; end).class_eval do
          alias_method orig_update_meth, :update
          def update(*args) ; end
        end
      end

    end
  end
end

tweet_observer.rb

class TweetObserver < ActiveRecord::Observer
  observe :post, :implementation, :question

  def after_create(model)
    tweet(model.tweet_title, model.tweet_path)
  end

  private

    def tweet(title, path)
      # stuff that we only want to run in production environment
    end

end

tweet_observer_spec.rb

describe TweetObserver do

  include RailsBestPractices::Spec::Support

  it 'should be observing Post#create' do
    Post.delete_all
    within_observable_scope do |observer|
      instance = Factory.build(:post, :id => 1)
      observer.should_receive(:tweet).with(instance.tweet_title, instance.tweet_path)
      instance.save
    end
  end

end