Flutter Cloud Firestore：如何检查字段是否存在

Question

我的 Flutter 应用使用 Firbase Cloudfirestore 作为其后端。稍后我想向我的应用程序添加新功能，这将需要 Firestore 文档中的新字段。如何检查文档中是否存在该字段，如果不存在则返回默认值？

Stream<List<Car>> streamCars() {
    return _carsCollection.snapshots().map((snapshot) => snapshot.docs.map((document) => Car.fromDocumentSnapshot(document)).toList());
}

static Car fromDocumentSnapshot(DocumentSnapshot snapshot) {
  return Car(
    id: snapshot.id,
    date: snapshot['date'] ?? Timestamp.now(),
    seats: snapshot['seats'] ?? 0,
    newFeature: snapshot['newFeature'] ?? '', // This field does not exist yet and throws error
  );
}

这会引发错误：

Bad state: field does not exist within the DocumentSnapshotPlatform

Answer 1

你可以这样做。

newFeature: (snapshot.data() as Map)['newFeature'] ?? ''

Answer 2

static Car fromDocumentSnapshot(DocumentSnapshot snapshot) {
  return Car(
    id: snapshot.id,
    date: snapshot['date'] ?? Timestamp.now(),
    seats: snapshot['seats'] ?? 0,
    newFeature: snapshot['newFeature']==null ?'tjhjhjh': '', // check it if is null will solve the error
  );
}