title: What is Firebase Firestore 'Reference' data type good for?
body: <p>I'm just exploring the new Firebase Firestore and it contains a data type called <a href="https://firebase.google.com/docs/firestore/manage-data/data-types" rel="noreferrer"><code>reference</code></a>. It is not clear to me what this does. </p>
  
  <ul>
  <li>Is it like foreign key? </li>
  <li>Can it be used to point to a collection that is located somewhere else?</li>
  <li>If <code>reference</code> is an actual reference, can I use it for queries? For example can I have a reference that points directly to the user, instead of storing the userId in a text field? And can I use this user reference for querying?</li>
  </ul>