firebase - 获取具有等于特定字符串的字段的 Firestore 文档

Question

我正在尝试获取集合中具有等于特定字符串的特定字段的文档。我正在建立一个 POS，我想获得特定城市的所有销售额。

final FirebaseFirestore _firestore = FirebaseFirestore.instance;
final CollectionReference _mainCollection = _firestore.collection('Sales');


  Stream<QuerySnapshot> readFeeds() {
    CollectionReference notesItemCollection =
    _mainCollection.where('seller_location', isEqualTo: "London").get();

    return notesItemCollection.snapshots();
  }

我收到这个错误:

A value of type 'Future<QuerySnapshot<Object?>>' can't be assigned to a variable of type 'CollectionReference<Object?>'.

我有投加投as CollectionReference<Object?>;但查询仍然无效。这就是我访问数据的方式:

  @override
  Widget build(BuildContext context) {
    return StreamBuilder<QuerySnapshot>(
      stream: readFeeds(),
      builder: (context, snapshot) {
        if (snapshot.hasError) {
          return Text('Something went wrong');
        } else if (snapshot.hasData || snapshot.data != null) {
          return ListView.separated(
            separatorBuilder: (context, index) => SizedBox(height: 16.0),
            itemCount: snapshot.data!.docs.length,
            itemBuilder: (context, index) {
              var noteInfo = snapshot.data!.docs[index];
              String docID = snapshot.data!.docs[index].id;
              String name = noteInfo['name'].toString();
              String price = noteInfo['price'].toString();
              String quantity = noteInfo['quantity'].toString();
              return Ink(
                decoration: BoxDecoration(
                  color: CustomColors.firebaseGrey.withOpacity(0.1),
                  borderRadius: BorderRadius.circular(8.0),
                ),
                child: ListTile(
                  shape: RoundedRectangleBorder(
                    borderRadius: BorderRadius.circular(8.0),
                  ),
                  onTap: () => Navigator.of(context).push(
                    MaterialPageRoute(
                      builder: (context) => EditScreen(
                        documentId: docID,
                        currentName: name,
                        currentPrice: price,
                        currentQuantity: quantity,
                      ),
                    ),
                  ),
                  title: Text(
                    name,
                    maxLines: 1,
                    overflow: TextOverflow.ellipsis,
                    style: TextStyle(color: Colors.white),
                  ),
                ),
              );
            },
          );
        }
        return Center(
          child: CircularProgressIndicator(
            valueColor: AlwaysStoppedAnimation<Color>(
              CustomColors.firebaseOrange,
            ),
          ),
        );
      },
    );
  }
}

Answer 1

您收到以下错误:

A value of type 'Future<QuerySnapshot<Object?>>' can't be assigned to a variable of type 'CollectionReference<Object?>'.

因为下面这行代码:

CollectionReference notesItemCollection =
    _mainCollection.where('seller_location', isEqualTo: "London").get();

这是有道理的，因为 get()函数返回 Future并且不是 CollectionReference目的。在 Dart 中无法创建这样的转换，因此会出现错误。

由于您使用的是 where()函数，返回的对象类型是Query .所以你的代码应该是这样的:

Query queryBySellerLocation =
    _mainCollection.where('seller_location', isEqualTo: "London");

一旦您正确定义了这个查询，您就可以执行 get() 调用，并收集结果:

queryBySellerLocation.get().then(...);