title: How to access root_path with url param (in host app, from rails engine)?
body: <h3>Setup</h3>
  
  <p>engine's <code>routes.rb</code>:</p>
  
  <pre class="lang-rb prettyprint-override"><code>MyEngine::Engine.routes.draw do
    get '/:guid', :to =&gt; 'foo_bar#index'
  end
  </code></pre>
  
  <p>mounted in host app via <code>mount MyEngine::Engine, :at =&gt; '/some_route'</code></p>
  
  <h3>Issue</h3>
  
  <p>Somewhere in a helper in the host app:</p>
  
  <pre class="lang-rb prettyprint-override"><code>generated_link = my_engine.root_path(some_guid_i_got)
  </code></pre>
  
  <p>results in </p>
  
  <pre><code>undefined method `root_path' for #&lt;ActionDispatch::Routing::RoutesProxy:0x00007ff443d405a0&gt;
  </code></pre>
  
  <p>I tried adding <code>include MyEngine::Engine.routes.url_helpers</code>, but that broke a bunch of other routes that aren't even related to the engine.
  Engine is working other than this url-helper, but I don't want to hard-code the url in.</p>
  
  <p>What's the proper way of generating the url for <code>/some_route/a-guid-here</code>?</p>
  
  <p>Thanks in advance</p>