How to merge two streams and emit them simultaneously using StreamBuilder - flutter

Question

I'm trying to make 2 simultaneous queries to Firebase and then stream their results as a single stream, emitting their results one at a time as they come through.
I've tried using this code below, but only one of the stream seems to work, even when both streams should have a result.
Stream<QuerySnapshot> searchQuery(String searchQuery) {
final firstStream = FirebaseFirestore.instance
.collectionGroup("newProduct")
.where('sWords', arrayContains: searchQuery)
.snapshots();
final secondStream = FirebaseFirestore.instance
.collectionGroup("usedProduct")
.where("sWords", arrayContains: searchQuery)
.snapshots();
final mergedStream = Rx.merge([firstStream, secondStream]);
return mergedStream;
}
This is the StreamBuilder where I'm making use of the merged stream
StreamBuilder(
stream: _marketDatabaseService.searchQuery(_searchQuery),
builder: (context, snapshot) {
if (snapshot.hasError) {
return Text("An error has occurred!");
} else if (snapshot.hasData &&
snapshot.connectionState == ConnectionState.active) {
List<QueryDocumentSnapshot> querySnapshot =
snapshot.data.documents;
return ListView.builder(
itemCount: querySnapshot.length,
itemBuilder: (BuildContext context, int index) {
return Text(querySnapshot[index]["prN"]);
});
} else {
return Center(
child: CircularProgressIndicator(
backgroundColor: Colors.blue,
),
);
}
},
),
Note: I'm making use of rxdart package in this example.

Answer 1

To query 2 collections in Firebase, use the following code snippet:
Stream<List<QuerySnapshot>> getData(String searchQuery) {
Stream stream1 = FirebaseFirestore.instance
.collectionGroup('newProdut')
.where('sWords', arrayContains: searchQuery)
.snapshots();
Stream stream2 = FirebaseFirestore.instance
.collectionGroup('usedProduct')
.where('sWords', arrayContains: searchQuery)
.snapshots();
return StreamZip([stream1, stream2]);
}
Then in your StreamBuilder Widget you the following code snippet to output the data:
StreamBuilder(
stream: getData(_searchQuery),
builder: (context, snapshot) {
if (snapshot.hasError) {
return Text("An error has occurred!");
} else if (snapshot.hasData &&
snapshot.connectionState == ConnectionState.active) {
List<QuerySnapshot> querySnapshot = snapshot.data.toList();
List<QueryDocumentSnapshot> documentSnapshot = [];
querySnapshot.forEach((query) {
documentSnapshot.addAll(query.docs);
});
/// This "mappedData" will contain contents from both streams
List<Map<String, dynamic>> mappedData = [];
for (QueryDocumentSnapshot doc in documentSnapshot) {
mappedData.add(doc.data());
}
return ListView.builder(
itemCount: mappedData.length,
itemBuilder: (context, index) {
return Text(mappedData[index]["prN"]);
});
} else {
return Center(
child: CircularProgressIndicator(
backgroundColor: Colors.blue,
),
);
}
},
),