title: Cloud firestore query of Map returning empty snapshot in Flutter/Dart
body: <p>I do a query to get a document by checking a Map key from firestore. I use a StreamBuilder for that. Both Id are the same in firestore and in flutter code. Do you have an idea why the snapshot is empty ? postSnapshot.data.documents.length is equal to 0, it returns [].</p>
  <pre><code>StreamBuilder(
          stream: Firestore.instance
              .collection('posts')
              .orderBy('createdAt', descending: true)
              .where('userlist.' + widget.user.uid, isEqualTo: true)
              .snapshots(),
          builder: (context, postSnapshot) {
            if (postSnapshot.connectionState == ConnectionState.waiting) {
              return _displaysLoading();
            }
            if (postSnapshot.hasData) {
             ...
            }
          }
      )
  </code></pre>
  <p><a href="https://i.stack.imgur.com/r4fES.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/r4fES.png" alt="enter image description here" /></a></p>