save-object-in-firstore.dart

static void saveOrUpdateVisitor(final Visitor visitor, final bool signIn) async {
    if (visitor != null) {
      // _firestore is a private variable declared using -> static Firestore _firestore = Firestore.instance;
      await _firestore.collection(Constants.visitors)/// create the collection where Visitors will be saved
          .document(visitor.emailAddress)/// construct a path (more like reference)
          .setData(Visitor.toMap(visitor, signIn));/// save data
    }
  }