title: how to get child key from parent key of a node on firebase
body: <p>i was trying to check if a date does exists on my db.
  <a href="https://i.stack.imgur.com/UIWVY.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/UIWVY.png" alt="enter image description here"></a></p>
  
  <p>lets say my date is 2018-04-23 and then i wanted to check if the date existed on my database so i use this</p>
  
  <pre><code>ref.once('value', function(snapshot){
          console.log(snapshot.val().key); // but this returns undefined. 
          console.log(body.datebook);
  
  })
  </code></pre>
  
  <p>i dont want to use forEach loop for a specific reason. is there a way to get the dates only?</p>