我是flutter的初学者，从Firebase数据库获取/获取数据时遇到问题

  void fetch() async {
    QuerySnapshot snapshot = await  _firestore.collection('users')
        .document(your uid)
        .collection('tasks').getDocuments();
    final documents = snapshot.documents;
    for(var doc in documents){
      print(doc.data['booking_date']);
    }
  }