Using Rails 4.1 Action Pack variants with Vendor MIME Type for simple API versioning

gistfile1.rb

# config/routes.rb
Rails.application.routes.draw do
  # use json if format not defined in url
  scope(format: :json) do
    get :hello, controller: :sandbox
  end
end

# config/initializers/mime_types.rb
# Define a custom Vendor MIME type 
Mime::Type.register "application/sandbox-v1+json", :v1json

# app/controllers/sandbox_controller.rb
class SandboxController < ApplicationController
  def hello
    respond_to do |format|
      format.json # hello.jbuilder
      format.v1json do 
        request.variant = :v1 # hello+v1.jbuilder
      end 
    end
  end
end

# app/views/sandbox/hello.jbuilder
json.hello "from default json"

# app/views/sandbox/hello+v1.jbuilder
json.hello "from v1 json variant"

# GET [Accept:'application/json', Content-Type:"application/json"] /sandbox/hello
# { "hello":"from default json" }

# GET [Accept:'application/sandbox-v1+json', Content-Type:"application/sandbox-v1+json"] /sandbox/hello
# { "hello":"from v1 json variant" }