在我的 rails3 引擎 gemfile 中，我添加了：
gem 'paperclip'
gem 'inherited_resources'
我还将它添加到我的 gemspec 中：
s.add_dependency "paperclip"
s.add_dependency "inherited_resources"
然后，我为我的引擎和客户端应用程序运行 bundle install 并启动客户端应用程序的控制台，以便首先测试回形针，我得到：
Using /Users/ynkr/.rvm/gems/ruby-1.9.2-p180 with gemset rails31beta
ynkr % rails c
/Users/ynkr/.rvm/gems/ruby-1.9.2-p180@rails31beta/gems/actionpack-3.1.0.rc4/lib/action_dispatch/http/mime_type.rb:101: warning: already initialized constant YML
Loading development environment (Rails 3.1.0.rc4)
ruby-1.9.2-p180 :001 > b=Blog::Blog.first
Blog::Blog Load (0.2ms) SELECT `blogs`.* FROM `blogs` LIMIT 1
=> #<Blog::Blog id: 1, user_id: 1, context_id: 2, title: "Cmd Line Blog Title", title_for_url: "cmd-line-blog-title", teaser: "This is the teaser for the command line blog", content: "Some content for the cmd line blog", created_at: "2011-06-28 06:06:55", updated_at: "2011-06-28 06:06:55">
ruby-1.9.2-p180 :002 > b.photos
NoMethodError: undefined method `has_attached_file' for #<Class:0x00000102a57fc8>
from /Users/ynkr/.rvm/gems/ruby-1.9.2-p180@rails31beta/gems/activerecord-3.1.0.rc4/lib/active_record/base.rb:1078:in `method_missing'
from /websites/gems/blog/app/models/blog/photo.rb:6:in `<class:Photo>'
from /websites/gems/blog/app/models/blog/photo.rb:1:in `<top (required)>'
from /Users/ynkr/.rvm/gems/ruby-1.9.2-p180@rails31beta/gems/activesupport-3.1.0.rc4/lib/active_support/dependencies.rb:452:in `load'
好的，所以似乎找不到回形针 gem 中的 has_attached_file 方法。为什么？我不知道。
继续讨论inherited_resources，我将脚手架生成的控制器更改为如下所示：
class Blog::ContextsController < InheritedResources::Base
before_filter :redirect_unless_admin
end
并加载我（在日志中）受到欢迎的索引页面：
Started GET "/blog/contexts" for 127.0.0.1 at 2011-06-28 12:06:25 -0700
ActionController::RoutingError (uninitialized constant InheritedResources):
Rendered /Users/ynkr/.rvm/gems/ruby-1.9.2-p180@rails31beta/gems/actionpack-3.1.0.rc4/lib/action_dispatch/middleware/templates/rescues/routing_error.erb within rescues/layout (0.5ms)
我的问题是为什么这些不起作用？我的直觉是，我完全不知道引擎、命名空间和加载 gem 的一些东西。