title: How to tell which descendants are changed with on("child_changed")
body: <p>For example, I have following database structure:</p>
  
  <pre><code>/ 
  + users
    + 1
      + items
        + -xxx: "hello"
    + 2
      + items
  </code></pre>
  
  <p>Then;</p>
  
  <pre><code>var usersRef = new Firebase("https://mydb.firebaseio.com/users");
  usersRef.on("child_changed", function(snapshot) {
    utils.debug(JSON.stringify(snapshot.exportVal()));
  });
  </code></pre>
  
  <p>If a value, "world", is pushed to "/users/1/items", I may get:</p>
  
  <pre><code>{"items": {"-xxx": "hello", "-yyy": "world"}}
  </code></pre>
  
  <p>So, how to tell which one is changed?</p>
  
  <p>I need to on("child_added") every single ref to "/users/$id/items"?</p>
  
  <p>NOTE: I'm trying to write an admin process in node.js.</p>