Why Does My Flutter Firebase 'whereIn' Query Return No Posts?

It’s likely that nothing is “wrong” with the groupId filter—it’s more that the query isn’t matching any documents because of how the additional query parameters (such as startAfterDocument) interact with the results. Here are a few things to check:

1. Exact Field Match:
   Make sure that the value stored in the Post document’s groupId field is exactly "oVF05dlEdzS3vdj3TvSG" (including type and whitespace). Even a small difference (capitalization, extra spaces, etc.) will cause the filter not to match.

2. Pagination with startAfterDocument:
   The use of startAfterDocument(_lastDocument!) means the query will return only posts coming after the document snapshot you provided. If that snapshot (from a previous query without the groupId filter) isn’t in the new filtered result set, then no documents will be returned. Try removing or commenting out the pagination temporarily to see if the document appears:

   DARTCopied
   QuerySnapshot snapshot = await FirebaseFirestore.instance
       .collection('Posts')
       .where('groupId', whereIn: ['oVF05dlEdzS3vdj3TvSG'])
       .orderBy('createdAtTimestamp', descending: true)
       //.startAfterDocument(_lastDocument!) // remove pagination for testing
       .limit(_numberOfPosts)
       .get();

   If this returns your document, then you know pagination is the culprit. In that case, update your pagination logic so that _lastDocument is based on the filtered query results.

3. Index Requirements:
   When combining a filter (even an equality filter via whereIn) with an orderBy, Firestore requires a composite index. However, if an index were missing, you’d normally see an error with a link to create the required index rather than getting back no results.

Test these items first. Adjusting your pagination logic or ensuring the document’s stored groupId exactly matches should resolve the issue.