Question

why my engine can’t be loaded?

When I startup server console show message:

`load_missing_constant': Expected D:/alexandre/dev/rails/project_name/lib/admin/engine.rb to define Admin::Engine (LoadError)

But “/lib/admin/engine.rb” exists and its contents is:

Admin::Engine.routes.draw do
    get '/' => "admin#index"
    get '/error' => "admin#error"

    post '/thunder' => "thunder#create", :as => "thunders"

    get '/:class' => "admin_models#index"
    get '/:class/new' => "admin_models#new"
    post '/:class' => "admin_models#create"
    get '/:class/:id' => "admin_models#show"
    get '/:class/:id/edit' => "admin_models#edit"
    put '/:class/:id' => "admin_models#update"
    delete '/:class/:id' => "admin_models#destroy"
end

In routes I add this line:

mount Admin::Engine => '/admin', :as => 'admin'

Project specification:

• ruby 1.9.2p180
• Rails 3.2.6

Answer 1

The file “/lib/admin/engine.rb”, should define the constant Admin::Engine, or more explicitely that file should define the class Admin::Engine. Here is a minimal example of the content for that file

module Admin
  class Engine < ::Rails::Engine
    #isolate_namespace Admin #see http://api.rubyonrails.org/classes/Rails/Engine.html#label-Isolated+Engine
  end
end

The snipped you posted before is about the routes of your engine and should therefore be placed in “config/routes.rb”

The documentation and the guide are great ways to get familiar with engines. If you like books, I advise you to read Crafting Rails Applications which is a small but very dense book and talks extensively about engines (and is written by one of the most awesome rubyist : Jose Valim).