title: ActiveAdmin - uninitialized constant AdminUser
body: <p>I'm getting an error while trying to run my app, having recently installed ActiveAdmin. It was working fine, but, after idling for a while, suddenly it isn't!</p>
  
  <p>In my gem file:</p>
  
  <pre><code>gem 'activeadmin'
  </code></pre>
  
  <p>In my routes:</p>
  
  <pre><code>  devise_for :admin_users, ActiveAdmin::Devise.config
  
    ActiveAdmin.routes(self) 
  </code></pre>
  
  <p>And in my admin_user.rb</p>
  
  <pre><code>ActiveAdmin.register AdminUser do     
    index do                            
      column :email                     
      column :current_sign_in_at        
      column :last_sign_in_at           
      column :sign_in_count             
      default_actions                   
    end   etc....
  </code></pre>
  
  <p>The error when trying to run:</p>
  
  <pre><code>NameError: uninitialized constant AdminUser
  ~/Desktop/Sites/Fleetnation/app/admin/admin_user.rb:1:in `&lt;top (required)&gt;'
  /usr/local/rvm/gems/ruby-1.9.3-p194/gems/activesupport-3.2.11/lib/active_support/inflector/methods.rb:230:in `block in constantize'
  /usr/local/rvm/gems/ruby-1.9.3-p194/gems/activesupport-3.2.11/lib/active_support/inflector/methods.rb:229:in `each'
  /usr/local/rvm/gems/ruby-1.9.3-p194/gems/activesupport-3.2.11/lib/active_support/inflector/methods.rb:229:in `constantize'
  ...
  </code></pre>
  
  <p>Anyone got any ideas? Would be much appreciated.</p>