title: How add elements to an existing map firestore
body: <p>i am doing some work with flutter and firebase, so i can't figure out how add new elements to an existing document who has an existing map with some created fields so.. the map for example is this </p>
  
  <pre><code>'Users':{uid0:{'Name':{'userName0'},uid1:{'Name':{'userName1'}}
  </code></pre>
  
  <p>An this is my actual code.</p>
  
  <pre><code>Firestore.instance.collection('on_going_matches').where('userSlot',isGreaterThan: 0).snapshots().listen(
            (data) =&gt; {
                Firestore.instance.runTransaction((transaction) async {
                  await transaction.update(Firestore.instance.collection("on_going_matches").document(data.documents[0].documentID), {
                    'users': {uid:{'Name':name}},
                    'userSlot': data.documents[0]['userSlot'] - 1
                  }
                  );
                })
                });
  </code></pre>
  
  <p>I have another question too, what i can do when "where" condition <code>"where('userSlot',isGreaterThan: 0)"</code> is not found, how i can use this like an if/else statement so, if Firestore query is not found, do a thing.</p>