title: Firebase security rules to check array length
body: <p>Is it possible to enforce the votes array to have exactly 5 entries?</p>
  
  <p>newData.child('votes').val().length === 5) doesn't seem to work as the length property only works with strings. Here is my data:</p>
  
  <pre><code>votes
     $voteID
       page:"12345"
       user: "facebook:1234567"
       votes: {
         -K3M2kpTEtXRk-sHV6FO: true
         -K3M2kp_5vZDCZKUke2M: true
         -K3M2kpe7l22mYA1dc6D: true
         -K3M2kqhpgehSSUAcFLl: true
         -K3M2krABl2tU-cekpDA: true
      }
  </code></pre>
  
  <p>Is it possible at all to enforce this validation ?</p>