title: FLUTTER | How to add data to an existing document in firestore
body: <p>I'm using firestore to store data of my flutter application, and I made a function that creates a document in firestore automatically  after the user login
  <a href="https://i.stack.imgur.com/RLFyG.jpg" rel="noreferrer">My Firestore database</a></p>
  
  <p>Now I want the user when he fills this form , the data will be added in the same document where the user's email exists.</p>
  
  <pre><code>[ RaisedButton(
                child: Text("Submit"),
                onPressed: () {
                 final CollectionReference users = Firestore.instance.collection('users');
                  Firestore.instance
                      .runTransaction((Transaction transaction) async {
                    CollectionReference reference =
                    Firestore.instance.collection('users');
  
                    await reference
                        .add({"fullname": nameController.text, "PhoneNumber": phoneController.text, "adresse": adressController.text});
                    nameController.clear();
                    phoneController.clear();
                    adressController.clear();
  
                  });},][2]
  </code></pre>
  
  <p>I tried this code but it adds new document.</p>