Rails versioned controllers via Accept header

app--controllers--v1-welcome_controller.rb

class V1::WelcomeController < V1::ApplicationController
  def index
    respond_to do |format|
      format.welcome { render plain: "testing v1" }
    end
  end
end

app--controllers--v2-welcome_controller.rb

class V2::WelcomeController < V1::ApplicationController
  def index
    respond_to do |format|
      format.welcome { render plain: "now working with v2" }
    end
  end
end

config--initializers--mime_types.rb

Mime::Type.register "application/vnd.welcome.v1", :welcome, ["application/vnd.welcome.v2"]

config--routes.rb

class VersionConstraint
  def initialize(version, default = false)
    @version = version
    @default = default
  end

  def matches?(request)
    # TODO use existing MIME type parsing code
    @default || request.headers['Accept'].include?("application/vnd.welcome.v#{@version}")
  end
end

Rails.application.routes.draw do
  def versioned(versions)
    last = versions.last
    versions.each do |version|
      version_module = "v#{version}".to_sym
      scope module: version_module, constraints: VersionConstraint.new(version, version == last) do
        yield
      end
    end
  end

  versioned(1..2) do
    get 'test', to: 'welcome#index'
  end
end