title: Using Stream Building with a specific Firestore document
body: <p>I am building a Flutter application and I am having trouble understanding how to implement Firestore. Out of the tutorials I have seen, I only see how to create a snapshot of an entire collection, however in my case, my collection is users, so I only need to snapshot the document of a particular user. There doesn't appear to be documentation on the Firebase docs on how to do this nor is there much documentation on the FlutterFire GitHub page. Please help!</p>
  
  <p>This is the Widget I'm trying to build with StreamBuilder.</p>
  
  <pre><code>  @override
    Widget build(BuildContext context) {
      return new StreamBuilder(
        stream: Firestore.instance.collection('users').document(userId).snapshots(),
        builder: (context, snapshot) {
          return new ListView.builder(
            itemCount: //what do I put here?,
            itemBuilder: (context, index) =&gt; new Item(//And here?),
          );
        }
      );
    }
  </code></pre>