title: Path helper for rails route, with param that mounts an engine
body: <p>Hi I have the following route:</p>
  
  <pre><code>mount Spree::Core::Engine, :at =&gt; ':city_name/', :as =&gt; :spree
  </code></pre>
  
  <p>that produces urls like <code>/munich/path/in/engine</code> or <code>/new_york/path/in/engine</code></p>
  
  <p>How can I use an url helper to produce such an url, that modifies the city and uses an url helper of the engine? If the engine would provide an url helper for a product (e.g. <code>product_path(@product)</code>), is there something like</p>
  
  <pre><code>spree_path(:city_name =&gt; "munich").product_path(@product)
  </code></pre>
  
  <p>to produce an url like <code>/munich/products/1</code>?</p>