获取字段等于 flutter 中特定字符串的 Firestore 文档

Question

I am trying to get documents in a collection that have a particular field that equals a particular string. I am building a POS and I want to get all the sales for a particular city.

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();
  }

I am getting this error:

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

I have cast added cast as CollectionReference<Object?>; but the query is still not working. This is how I am accessing the data:

  @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

You are getting the following error:

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

Because of the following line of code:

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

Which makes sense, since the get() function returns aFuture and not a CollectionReference object. There is no way in Dart in which you can create such a casting, hence that error.

Since you are using the where() function, the type of the object that is returned is Query . So your code should look like this:

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

Once you have this query correctly defined, you can perform the get() call, and collect the results:

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

Answer 2

Please try this if it helps.

QuerySnapshot<Map<String,dynamic>>readFeeds() {
   QuerySnapshot<Map<String,dynamic>>response=await   _mainCollection.where('seller_location', isEqualTo: "London").get()

return response;
  }

you can access this data like this

response.docs.forEach((element) {
        ///this is Map<String,dynamic>
        element.data();
      });