Rails 4 - Respond only to JSON and not HTML

I believe there are 2 parts here:
1) json only requests in rails
2) json only responses in rails

1) Configure your application controller to ensure json requests only

# app/controller/application_controller.rb
before_action :ensure_json_request

def ensure_json_request
  return if request.format == :json
  render :nothing => true, :status => 406
end
2) Configure your Rails API routes to ensure json responses only

# config/routes.rb
MyApp::Application.routes.draw do
  namespace :api, constraints: { format: 'json'  } do
    namespace :v1 do
      resources :posts
    end
  end
end