module ActionDispatch::Routing::Mapper::Concerns

def concerns(*args)

end
concerns :commentable
namespace :posts do

Concerns also work in any routes helper that you want to use:

end
concerns :commentable
resources :posts do

Use the named concerns 
def concerns(*args)
  options = args.extract_options!
  args.flatten.each do |name|
    if concern = @concerns[name]
      concern.call(self, options)
    else
      raise ArgumentError, "No concern named #{name} was found!"
    end
  end
end