# config/initializers/routing_draw.rb
class ActionDispatch::Routing::Mapper
  def draw(routes_name, sub_path=nil)
    if sub_path.present?
      instance_eval(File.read(Rails.root.join("config/routes/#{sub_path}/#{routes_name}.rb")))
    else
      instance_eval(File.read(Rails.root.join("config/routes/#{routes_name}.rb")))
    end
  end
end

# routes.rb
Rails.application.routes.draw do
  draw :api
  resources :users
  root to: 'users#index'
end

# config/routes/api.rb
require 'api_constraints'

namespace :api, compress: false do
  [:v1, :v2].map { |api| draw api, :api }
end

# config/routes/api/v1.rb
scope module: :v1, constraints: ApiConstraints.new(version: 1, default: false) do
  resources :users
end