gistfile1.rb

class ActionDispatch::Routing::Mapper
  def draw(routes_name)
    instance_eval(File.read(Rails.root.join("config/routes/#{routes_name}.rb")))
  end
end

BCX::Application.routes.draw do
  draw :api
  draw :account
  draw :session
  draw :people_and_groups
  draw :projects
  draw :calendars
  draw :legacy_slugs
  draw :ensembles_and_buckets
  draw :globals
  draw :monitoring
  draw :mail_attachments
  draw :message_preview
  draw :misc

  root to: 'projects#index'
end

@sharipov-ru, it's much better but I'd like to add a small improvement:
ActiveSupport::FileUpdateChecker.new([], 'config/routes' => 'rb') do ... end
This way it should find new files as well.

Guys, it's very outdated. You now can just require other routes files without monkey patching, they just have to repeat the Rails.application.routes.draw do:
config/routes.rb:

Rails.application.routes.draw do
  load Rails.root.join("config/routes/dev.rb")
  #...
end

config/routes/dev.rb:

Rails.application.routes.draw do
  scope "dev" do
    # ...
  end
end

No dynamic reloading tho, probably @sharipov-ru's reloader still works.

Anyone found a nice solution on how to seperate route files and still make them auto reloading during development?

Edit: Going now with a combination of the mentioned solutions...

# config/environments/development.rb
class RoutesReloader
  def initialize(app)
    @app = app

    @routes_reloader = ActiveSupport::FileUpdateChecker.new([], 'config/routes' => 'rb') do
      Rails.application.reload_routes!
    end
  end

  def call(env)
    @routes_reloader.execute_if_updated

    @app.call(env)
  end
end

Rails.application.routes.draw do
  ...
  config.middleware.use RoutesReloader
end

# config/routes.rb
Rails.application.routes.draw do
  routes = [:admin_routes, :admin_routes_old, :public_routes, :api_routes]
  routes.each{ |route_file| load Rails.root.join("config", "routes", "#{route_file}.rb") }
end

How can I use concern with this conven?
https://guides.rubyonrails.org/routing.html#routing-concerns

concern :commentable do
  resources :comments
end
resources :messages, concerns: :commentable

can I do this?

draw :api, concerns: :commentable

or there another way?