title: Active admin not showing all input fields just last one
body: <p>I have this code inside app/admin/products.rb. I have problem with that I can see just one input field at time(the last one listed) or just submit button. Does it could be because of some syntax mistakes ? But there is no any error messages.</p>
  
  <pre><code>ActiveAdmin.register Product do 
     f.input :name,:label =&gt; "Name"
     f.input :photo, :as =&gt; :file
     f.input :category, :collection =&gt; @category
     f.input :manufacturer, :collection =&gt; @manufacturer
     f.actions do
     f.action :submit, :button_html =&gt; { :class =&gt; "primary", :disable_with =&gt; 'Wait...' } 
   end 
  end
  end
  </code></pre>
  
  <p>Product model looks like this</p>
  
  <pre><code> attr_accessible :category_id, :description, :manufacturer_id, :name, :photo 
   extend FriendlyId
  has_attached_file :photo,
   :styles =&gt; {
     :thumb=&gt; "100x100#",
     :large =&gt; "290x170",
      :medium=&gt; "120x120"} 
  friendly_id :name, use: [:slugged, :history]
  belongs_to :manufacturer
  belongs_to :category
  </code></pre>