How to merge two firestore queries in the Flutter project and How do I remove the duplicate documents from these two streams?

Question

I needed to combine two firestore query stream in my flutter project. How do I do this? I tried StreamZip([Stream1, stream2]) method to combine the streams and it worked for me. but the streams maybe contains the same documents. so when I listed them all of the documents are listed, even there is a duplicate of it. How do I remove the duplicate documents from these two streams?

   Stream<List<QuerySnapshot>> getData() {
    Stream defaultStream1 = _firestore
        .collection("Gyms")
        .where("gymPlaceTags", arrayContainsAny: ["dubai"])
        .orderBy('createdAt', descending: true)
        .snapshots();
    Stream defaultStream2 = _firestore
        .collection("Gyms")
        .where("gymFavTags", arrayContainsAny: ["ajman"])
        .orderBy('createdAt', descending: true)
        .snapshots();
    return StreamZip([defaultStream1, defaultStream2]);
  }

Answer 1

There are several steps you'll need to take:

1. map the stream to return another a List<DocumentSnapshot> instead of List<QuerySnapshot>
2. In the map function, use fold on the List to remove duplicates and map to List<DocumentSnapshot>

3. In the fold function, go over every document in the QuerySnapshot and check if the document with the same id is already present before adding.

   Stream<List<DocumentSnapshot>> merge(Stream<List<QuerySnapshot>> streamFromStreamZip) {
     return streamFromStreamZip.map((List<QuerySnapshot> list) {
       return list.fold([], (distinctList, snapshot) {
         snapshot.documents.forEach((DocumentSnapshot doc) {
           final newDocument = distinctList.firstWhere(
                   (DocumentSnapshot listed) =>
                       listed.documentId == doc.documentId,
                   orElse: () => null) == null;
           if (newDocument) {
             distinctList.add(doc);
           }
         });
         return distinctList;
       });
     });
   }
   

   Note: I didn't run this code, but it should be something like this.