title: How to use startAt() in Firebase query?
body: <p><a href="https://i.stack.imgur.com/7NDrV.png" rel="noreferrer"><img src="https://i.stack.imgur.com/7NDrV.png" alt="enter image description here"></a></p>
  
  <p>Let's suppose above firebase database schema. </p>
  
  <p>What I want to is retrieve messages which after "15039996197" timestamp. Each of message object has a <code>createdAt</code> property. </p>
  
  <p>How can I get messages only after specific timestamp? In this case, last two messages are what I want to retrieve. </p>
  
  <p>I tried <code>firebaseb.database().ref(`/rooms/$roomKey/messages`).startAt('15039996197')</code> but failed. It return 0. How can I do this?</p>