concern_sample.rb

module C
  extend ActiveSupport::Concern

  module ClassMethods
    def c
      "c"
    end
  end
end

module B
  extend ActiveSupport::Concern
  include C

  module ClassMethods
    def b
      "b" + c
    end
  end
end

class A
  include B

  class << self
    def a
      p "a" + b
    end
  end
end

if __FILE__ == $0
  A.a
end