title: parent() method in the security api for firebase
body: <p>I would like to set the following rule in firebase security.
  To authorize write, I would like to make sure that the auth.user_id is the same as RulesDataSnapshot called "user_id" under the parent of the current branch</p>
  
  <p>In order to do so, I tried writing the following write_rule:</p>
  
  <pre><code>".write": "auth.user_id == (data.parent().child('user_id').val())"
  </code></pre>
  
  <p>Firebase security API has a section on parent() method. 
  <a href="https://www.firebase.com/docs/security/rulesdatasnapshot/parent.html" rel="nofollow">https://www.firebase.com/docs/security/rulesdatasnapshot/parent.html</a></p>
  
  <p>I used the parent() method as shown in the example. However, I recieve an error " No such method 'parent' "</p>
  
  <p>Any help would be appreciated</p>