title: flutter firestore, add new object in array
body: <p>I have array of objects, I want to add new object when user enter new data in the array?</p>
  
  <pre><code>Firestore.instance.collection(city).document('Attractions').updateData(
                          "data", FieldValue.arrayUnion(obj)
                        );
  </code></pre>
  
  <p>This shows error, How can I achieve this with flutter?</p>