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