flutter-firestore：如何检索地图字段和数组字段

无法读取文档中字段的子集。您必须获取整个文档，然后使用点符号或 FieldPath 来获取嵌套数据：

    FirebaseFirestore.instance
    .collection('users')
    .doc(userId)
    .get()
    .then((DocumentSnapshot documentSnapshot) {
      if (documentSnapshot.exists) {
        try {
          dynamic nested = snapshot.get(FieldPath(['g', 'geopoint']));
          } on StateError catch(e) {
            print('No nested field exists!');
          }
      } else {
        print('Document does not exist on the database');
      }
    });