title: constraints in ruby-on-rails routing
body: <p>Could someone describe what this is all about?</p>
  
  <p>It's in the routing file:</p>
  
  <pre><code>match "photo", :constraints =&gt; {:subdomain =&gt; "admin"}
  </code></pre>
  
  <p>I can't understand it.</p>
  
  <p>thanks</p>