1_widget.rb

class Widget < ActiveRecord::Base
  include Thingable
end

2_thing.rb

class Thing < ActiveRecord::Base
  belongs_to :widget
end

3_thingable.rb

#thingable is namespaced to Widget
module Widget::Thingable
  extend ActiveRecord::Concern

  included do
    has_many :things
  end
end

4_console.txt

#tries to load Thing namespaced to the same thing as the concern is namespaced to
> widget.things
uninitialized constant Widget::Thing

5_workaround.rb

# forces rails not to use concern namespace
# this workaround works
module Widget::Thingable
  extend ActiveRecord::Concern

  included do
    has_many :things, class_name: Thing
  end
end