Rails: Including a Concern with a constant within a Concern

It appears this behavior is by design, as explained nicely over here.

What you'll need to do in this case is not have Group::Constants extend from ActiveSupport::Concern since that will block its implementation from being shared with other ActiveSupport::Concern extending modules (although it will be ultimately shared in a class that includes the second module):

module A
  TEST_A = 'foo'
end

module B
  extend ActiveSupport::Concern
  TEST_B = 'bar'
end

module C
  extend ActiveSupport::Concern
  include A
  include B
end

C::TEST_A 
=> 'foo'
C::TEST_B 
=> uninitialized constant C::TEST_B

class D
  include C
end

D::TEST_A 
=> 'foo'
D::TEST_B 
=> 'bar'

In short, you'll need to make Group::Constants a standard module and then all will be well.

If you want to keep everything in one file, and if you can stomach a bit of boilerplate, you could break up your module into a "concern" bit and a "non-concern" bit:

module A
  FOO = [22]

  def self.included base
    base.include Concern
  end

  module Concern
    extend ActiveSupport::Concern

    class_methods do
      def foo_from_the_perspective_of_a_class_method_in_A
        {lexical: FOO, instance: self::FOO}
      end
    end
  end
end

module B
  extend ActiveSupport::Concern

  include A

  FOO += [33]

  def foo_from_the_perspective_of_an_instance_method_in_B
    FOO
  end
end

class C
  include B
end


C.foo_from_the_perspective_of_a_class_method_in_A
=> {:lexical=>[22], :instance=>[22, 33]}
C.new.foo_from_the_perspective_of_an_instance_method_in_B
=> [22, 33]
C::FOO
=> [22, 33]