title: Draper gem not decorating associated model
body: <p>I'm following the Draper gem documentation, but the associated model is not being decorated. Here's a snippet of my top-level decorator:</p>
  
  <pre><code>module Admin
    class OrderDecorator &lt; Draper::Decorator
      delegate_all
      decorates_finders
      decorates_association :line_item
  
      def amount
        model.amount.nil? ? 0 : model.amount
      end
    end
  end
  </code></pre>
  
  <p>And here's a snippet from my association decorator:</p>
  
  <pre><code>module Admin
    class LineItemDecorator &lt; Draper::Decorator
      delegate_all
  
      def options
        model.options.present? ? model.options.to_s : 'N/A'
      end
    end
  end
  </code></pre>
  
  <p>Here's the method in the controller that calls the decorator:</p>
  
  <pre><code>  def show
      @subject = Admin::OrderDecorator.find(params[:id])
    end
  </code></pre>
  
  <p>It's being used in the view like so:</p>
  
  <pre><code>@subject.line_items.each do |li|
    = li.options
  </code></pre>
  
  <p>...etc...</p>
  
  <p>The options method on LineItemDecorator is just being ignored - if it's nil, it doesn't display anything, rather than 'N/A'. I've put binding.pry in the options method and it never gets called. Can anyone see what I'm missing?</p>
  
  <p>Thanks so much in advance.</p>