observed_classes get redefined properly - fixes #3505

[frodsan]

I'm trying to fix #3505 issue:

I need help with this:

rails/activemodel/lib/active_model/observing.rb:199: warning: previous definition of observed_classes was here
rails/activemodel/lib/active_model/observing.rb:199: warning: method redefined; discarding old observed_classes

I'm getting those warnings in this test:

test "passes observers to subclasses" do
    FooObserver.instance
    bar = Class.new(Foo)
    assert_equal Foo.observers_count, bar.observers_count
end

I don't know how to check if the method is already defined in the subclass?

  def observe(*models)
    models.flatten!
    models.collect! { |model| model.respond_to?(:to_sym) ? model.to_s.camelize.constantize : model }

    unless self.singleton_class.respond_to?(:observed_classes)
      define_singleton_method(:observed_classes) { models }
    end
  end

[jeremyf]

@frodsan There are failing tests:

./activerecord/test/cases/lifecycle_test.rb "test_auto_observer"

[marcandre]

Your patch won't work in cases where observe changes again; see the patch I created (I was working in Observing today)

[frodsan]

@marcandre nice :)