title: how to select a doc from a firebase according to their time of creation?
body: <p>I am learning the firebase and currently working on the project as well. I m bad at setting queries through the Database.</p>
  <p>here is my firebase db structure</p>
  <p><img src="https://i.stack.imgur.com/5vpZ6.png" alt="Image" /></p>
  <p>What I want is to select the newest document (according to their time of creation Or 'created' variable in them ) from the subscriptions' collection.</p>