Enabling/disabling observers for testing

Wed, 09 Apr 2008 19:53:50 -0700
ruby, rails, observers, testing

If you use ActiveRecord observers in your application and are concerned about the isolation of your model unit tests, you probably want some way to disable/enable observers. Unfortunately, Rails doesn't provide an easy way to do this. So, here's some code I threw together a while ago to do just that.

module ObserverTestHelperMethods
  def observer_instances
    ActiveRecord::Base.observers.collect do |observer|
      observer_klass = \
        if observer.respond_to?(:to_sym)
          observer.to_s.camelize.constantize
        elsif observer.respond_to?(:instance)
          observer
        end
      observer_klass.instance
    end
  end

  def observed_classes(observer=nil)
    observed = Set.new
    (observer.nil? ? observer_instances : [observer]).each do |observer|
      observed += (observer.send(:observed_classes) + observer.send(:observed_subclasses))
    end
    observed
  end

  def observed_classes_and_their_observers
    observers_by_observed_class = {}
    observer_instances.each do |observer|
      observed_classes(observer).each do |observed_class|
        observers_by_observed_class[observed_class] ||= Set.new
        observers_by_observed_class[observed_class] << observer
      end
    end
    observers_by_observed_class
  end

  def disable_observers(options={})
    except = options[:except]
    observed_classes_and_their_observers.each do |observed_class, observers|
      observers.each do |observer|
        unless observer.class == except
          observed_class.delete_observer(observer)
        end
      end
    end
  end

  def enable_observers(options={})
    except = options[:except]
    observer_instances.each do |observer|
      unless observer.class == except
        observed_classes(observer).each do |observed_class|
          observer.send :add_observer!, observed_class
        end
      end
    end
  end
end

Include this in a Test::Unit::TestCase or 'include' in your RSpec configuration, whatever rocks your boat. Here's a stupid example:

class SomethingCoolTest < Test::Unit::TestCase
  include ObserverTestHelperMethods

  def setup
    disable_observers
  end

  def teardown
    enable_observers
  end

  def test_without_observers
    # ...
  end

end

When you go to test the behavior of the observer itself, simply disable/enable like the following to disable/enable all observers except the one you're testing:

class DispassionateObserverTest < Test::Unit::TestCase
  include ObserverTestHelperMethods

  def setup
    disable_observers :except => DispassionateObserver
  end

  def teardown
    enable_observers :except => DispassionateObserver
  end

  def test_without_observers_except_dispassionate_observer
    # ...
  end

end