title: Rails routing - :on => :collection
body: <p>The <a href="http://guides.rubyonrails.org/routing.html">Rails routing guide</a> doesn't specify what <code>:on =&gt; :collection</code> means.</p>
  
  <p>I cannot find an explanation to what the <code>:on</code> key is, nor what a <code>:collection</code> is in that context.</p>