如何从 firebase 获取数据并从中创建类对象答案

【问题标题】：How to fetch data from firebase and create a class object from it如何从 firebase 获取数据并从中创建类对象
【发布时间】：2022-11-04 02:50:09
【问题描述】：

我正在尝试从 firebase 获取用户评分并从数据中创建一个类。

这是数据到达时我需要创建的类::

@immutable
class UserRating {
  final String documentId;
  final String ratingJobId;
  final String ratingUserIdGivingRating;
  final String ratingUserIdGettingRating;
  final int? ratingUser;
  final String? ratingComment;
  const UserRating(
      {required this.documentId,
      required this.ratingJobId,
      required this.ratingUserIdGivingRating,
      required this.ratingUserIdGettingRating,
      this.ratingUser,
      this.ratingComment});

  // acts as constructor
  UserRating.fromSnapshot(QueryDocumentSnapshot<Map<String, dynamic>> snapshot)
      : documentId = snapshot.id,
        ratingJobId = snapshot.data()[ratingJobIdColumn],
        ratingUserIdGivingRating =
            snapshot.data()[ratingUserIdGivingRatingColumn],
        ratingUserIdGettingRating =
            snapshot.data()[ratingUserIdGettingRatingColumn],
        ratingUser = snapshot.data()[ratingUserColumn],
        ratingComment = snapshot.data()[ratingCommentColumn];
}

这是从firebase发出请求的代码（这是问题所在，因为我很难处理快照）

我遇到的另一个问题是数据可能还不存在，所以如果没有 userRating，我需要返回一些东西....也不知道如何处理。任何帮助都会很棒！

  final userRating = FirebaseFirestore.instance.collection('user rating');

 Future getUserRatingPerJob({
    required String ratingJobId,
    required String ratingUserIdGivingRating,
  }) async {
    await userRating
        .where(
          ratingJobIdColumn,
          isEqualTo: ratingJobId,
        )
        .where(
          ratingUserIdGivingRatingColumn,
          isEqualTo: ratingUserIdGivingRating,
        ); // this is where I need to do something to return UserRating object
  }

【问题讨论】：

标签： flutter firebase dart future snapshot

【解决方案1】：

这就是我要工作的。

有更清洁的方法吗？

  Future getUserRatingPerJob({
    required String ratingJobId,
    required String ratingUserIdGivingRating,
  }) async {
    var _userRating;

    await userRating
        .where(
          ratingJobIdColumn,
          isEqualTo: ratingJobId,
        )
        .where(
          ratingUserIdGivingRatingColumn,
          isEqualTo: ratingUserIdGivingRating,
        )
        .limit(1)
        .get()
        .then((value) {
      value.docs.forEach((element) {
        _userRating = CloudUserRating(
          documentId: element.id,
          ratingJobId: element[ratingJobIdColumn],
          ratingUserIdGettingRating: element[ratingUserIdGettingRatingColumn],
          ratingUserIdGivingRating: element[ratingUserIdGivingRatingColumn],
          ratingUser: element[ratingUserColumn],
          ratingComment: element[ratingCommentColumn],
        );
      });
    });
    return _userRating;
  }

【讨论】：