尝试从 Firebase FireStore 文档中检索数据时出错

Question

我正在尝试检索用户已注册的所有课程，这些课程存在于文档中的数组中。

从 users 集合中检索课程 ID 后，我正在尝试从 courses 集合中检索课程详细信息。

但在填充courses 变量之前，会执行coursesCollection 语句并抛出以下错误。

======== Exception caught by widgets library =======================================================
The following assertion was thrown building _BodyBuilder:
'in' filters require a non-empty [List].
'package:cloud_firestore/src/query.dart':
Failed assertion: line 706 pos 11: '(value as List).isNotEmpty'

这是导致错误的代码：

 List courses = [];
  var coursesCollection;

  void fetchCourses() async {
    final loggedInUser = FirebaseAuth.instance.currentUser;
    if (loggedInUser != null) {
      final userCollection = await FirebaseFirestore.instance.collection('users').doc(loggedInUser.uid).get();
      courses = userCollection.get('coursesEnrolled');
    }
  }

  @override
  void initState() {
    fetchCourses();
    coursesCollection = FirebaseFirestore.instance.collection('courses').where('courseID', whereIn: courses);
    super.initState();
  }

  Widget build(BuildContext context) {
    return StreamBuilder<QuerySnapshot>(
      stream: coursesCollection.snapshots(),
      builder: (context, snapshot) {
        if (!snapshot.hasData) {
          if (snapshot.connectionState == ConnectionState.waiting) {
            return Center(
              child: CircularProgressIndicator(backgroundColor: kBrandColor),
            );
          }
        }
        final courseListStream = snapshot.data!.docs.map((course) {
          return CourseData.fromDocument(course);
        }).toList();

        List<BadgedCourseCard> courseCards = [];
        for (var course in courseListStream) {
          final courseDocID = course.courseDocID;
          final courseID = course.courseID;
          final courseTitle = course.courseTitle;
          final courseImage = course.courseImage;
          final courseBgColor = hexToColor(course.courseBackgroundColor.toString());
hexToColor(course.courseFgColor.toString());
          final badgedCourseCard = BadgedCourseCard(
            courseTitle: courseTitle.toString(),
            courseTitleTextColor: courseFgColor,
            cardBackgroundColor: courseBgColor,
            courseImage: courseImage.toString(),
            courseCardTapped: () {
              Provider.of<CourseProvider>(context, listen: false).currentCourseDetails(
                currentCourseDocID: courseDocID,
                currentCourseID: courseID,
              );
              Navigator.of(context).push(ScaledAnimationPageRoute(CourseLandingPage(courseID: courseID.toString())));
            },
            courseBookmarkTapped: () => print("Course Bookmark Tapped"),
            rightPadding: 3,
            bottomPadding: 0.5,
            cardWidth: 80,
          );
          courseCards.add(badgedCourseCard);
        }
        return SizedBox(
          height: 20.5.h,
          child: ListView(
            physics: BouncingScrollPhysics(),
            scrollDirection: Axis.horizontal,
            children: courseCards,
          ),
        );
      },
    );
  }

我该如何解决这个问题？

Answer 1

这里，

coursesCollection = FirebaseFirestore.instance.collection('courses').where('courseID', whereIn: courses);

courses 将是 []，因为 fetchCourses 是一个 async 调用。

将fetchCourses 的返回类型从void 更改为Future<void> 并尝试使用then 回调：

@override
void initState() {
  super.initState();
  fetchCourses().then((val) {
    coursesCollection = FirebaseFirestore.instance.collection('courses').where('courseID', whereIn: courses);
    setState(() {});
  });
}

我还建议使用FutureBuilder 作为更好的选择。

coursesCollection 为空，这就是您收到另一个错误的原因。仅当coursesCollection 不为空时才渲染StreamBuilder。

coursesCollection != null ? StreamBuilder(...) : SizedBox(),

为了收听用户注册的课程，可以使用另一个 StreamBuilder。这将是一个嵌套的 StreamBuilder 设置。

StreamBuilder<DocumentSnapshot<Map<String, dynamic>>>(
  stream: FirebaseFirestore.instance.collection('users').doc(FirebaseAuth.instance.currentUser.uid).snapshots(),
  builder: (context, snapshot) => snapshot.hasData ? StreamBuilder<QuerySnapshot<Map<String, dynamic>>>(
    stream: FirebaseFirestore.instance.collection('courses').where('courseID', whereIn: snapshot.data!.data()!['coursesEnrolled']).snapshots(),
    builder: (context, snapshotTwo) {},
  ) : Text('Loading...'),
),