title: How to get document with reference fields populated
body: <p>Consider posts document schema in firestore: </p>
  
  <pre><code>{
    user: "/users/userId", //reference to document in users collection
    description: "lorem ipsum" //string
  }
  </code></pre>
  
  <p>how can I get this document with user field populated with referred document data?
  this code <code>db.collection("posts").doc("postId").get().then(post =&gt; console.log(post))</code> returns post document.</p>