title: Can't use "acts_as_url" in ruby on rails 3
body: <p>I'm newb and I'm sorry because of my dumb question! Please, help me!!
  I'm working with Rails 3, and this my problem:</p>
  
  <p>I have a model name: Photo using gem 'mongoid'.</p>
  
  <p>I want to make a permalinks which are readable url instead of unreadable '_id' generated from mongoid!</p>
  
  <p>After searching in google, I found a gem called 'stringex'! I decided to use this gem, and put this line in my Gemfile: </p>
  
  <pre><code>gem 'stringex'
  </code></pre>
  
  <p>then ran "bundle install' to use it.</p>
  
  <p>I just do everything following the guide in github <a href="https://github.com/rsl/stringex" rel="nofollow">Readme_rsl/stringex</a> but the trouble occurs:</p>
  
  <pre><code>undefined method `acts_as_url' for Photo:Class
  </code></pre>
  
  <p>Is it because ROR 3 doesn't support this gem? Or I missed something? 
  Please, I need help!</p>