how to combine two Firestore collections? - flutter

Question

Example code
toys = toysRef.where('postState', isEqualTo: 'recruiting').orderBy('createdAt', descending: true).snapshots();
fruits = fruitsRef.where('postState', isEqualTo: 'recruiting').orderBy('createdAt', descending: true).snapshots();
hi I'm trying to combine or merge two streams
I want to show toys and fruits order by desc with one StreamBuilder Widget
There is a common field ['createdAt']
I want to implement like this.
if some item added to firestore , it should show on realtime.

Answer 1

You can use StreamGroup from rxdart to merge your toys and fruits.
StreamGroup<List<QuerySnapshot>> combinedStreams = StreamGroup<List<QuerySnapshot>>.merge([
toysRef.where('postState', isEqualTo: 'recruiting').orderBy('createdAt', descending: true).snapshots(),
fruitsRef.where('postState', isEqualTo: 'recruiting').orderBy('createdAt', descending: true).snapshots()
]);
and to finally have them sorted in a stream, you could have:
StreamBuilder<List<QuerySnapshot>>(
stream: combinedStreams,
builder: (BuildContext context, AsyncSnapshot<List<QuerySnapshot>> snapshot) {
List<QueryDocumentSnapshot> combinedData = [];
for (QuerySnapshot snap in snapshot.data) {
combinedData.addAll(snap.docs);
}
combinedData.sort((a, b) => b['createdAt'].compareTo(a['createdAt']));
return ListView.builder(
itemCount: combinedData.length,
itemBuilder: (context, index) {
// Build your UI
}
);
}
)
You should use ListView.builder so that you have a performant list, and not render all the elements at once (would really lag your UI if you'd have a lot of elements).
Don't forget to check if snapshot has errors or is empty so that you can display a loader to the user or an error message.