title: Can't render partial with interpolated instance variable
body: <p>I'm trying to render a partial on product pages that have the table column "group" set to "women".</p>
  
  <p>In products_controller.rb I have:</p>
  
  <pre><code>@subnav = Spree::Product.find(params[:group])
  </code></pre>
  
  <p>Then in products#show I have:</p>
  
  <pre><code>&lt;div class="subnav"&gt;
   &lt;%= render "spree/shared/#{@subnav}_subnav" %&gt;
  &lt;/div&gt;&lt;!--subnav --&gt;
  </code></pre>
  
  <p>So when a product has group "women" it should render "spree/shared/women_subnav". But instead I get a "ActionView::MissingTemplate in Spree/products#show" error, saying </p>
  
  <pre><code> "Missing partial spree/shared/_subnav with {:locale=&gt;[:en], :formats=&gt;[:html], :handlers=&gt;[:erb, :builder, :coffee, :rabl], :versions=&gt;[:v1]}.
  </code></pre>
  
  <p>So for some reason it's not even recognizing the existence of the variable.</p>
  
  <p>I've also tried </p>
  
  <pre><code> @subnav = params[:group]
  </code></pre>
  
  <p>and </p>
  
  <pre><code> @subnav = Product.find(params[:group]
  </code></pre>
  
  <p>Thanks for your help!</p>