title: Query a Map containing an ID saved in Cloud Firestore with Dart/Flutter
body: <p>How to get a Map stored in Cloud Firestore with Dart/Flutter ?
  I tried this but it only works with Array :</p>
  <pre><code>Firestore.instance
              .collection('posts')
              .orderBy('createdAt', descending: true)
              .where(&quot;userlist&quot;, arrayContains: userId)
              .snapshots(),
  </code></pre>
  <p>here is the map stored in firebase</p>
  <p><a href="https://i.stack.imgur.com/8EcDT.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/8EcDT.png" alt="enter image description here" /></a></p>