title: What is @id as passed to $resource?
body: <pre><code>$resource("/entries/:id", {id: "@id"}, {update: {method: "PUT"}})
  </code></pre>
  
  <p>What is @id?  </p>
  
  <p>On the $resource <a href="http://docs.angularjs.org/api/ngResource.$resource" rel="noreferrer">doc page</a> someone says this below, but I still don't understand.</p>
  
  <blockquote>
    <p>If the parameter value is prefixed with @ then the value of that
    parameter is extracted from the data object (useful for non-GET
    operations)." The data object here refers to the <code>postData</code>object if
    non-GET "class" action is used, or the instance itself if non-GET
    instance action is used.</p>
  </blockquote>