Remove referenced document from array Field list type in a document - flutter

Question

I have a collection in firebase called "community" and "users". All user records have a field "joinedCommunity" (a list of all joined communities).
I'm trying to figure a code that when a community is deleted, all user records are updated to only remove the community reference from "joinedCommunity" field list.
building this in flutterflow using custom action
onTap on a button in UI, the code is included as one of the actions before community document is deleted.
Future userRecordUpdate(DocumentReference community) async {
final instance = FirebaseFirestore.instance;
final batch = instance.batch();
var collection = instance.collection('users');
batch.update(collection, {
"joinedCommunity": FieldValue.arrayRemove([community])
});
await batch.commit();
}

Answer 1

You're using a CollectionReference, when what you want is a DocumentReference. As per the documentation, WriteBatch.update only works on a DocumentReference.
I have a few suggestions:
Try updating the field without using a WriteBatch. Use a for loop and a regular DocumentReference.update() call.
Then, update your code to use a WriteBatch to update the field. Also, keep in mind a batch is limited to 500 operations.
Finally, consider the security implications of allowing a client to be able to update any User document. You should probably update your security rules so that a user document can only be modified by that user. This code is probably something that should run in a Firebase Cloud Function that gets triggered whenever a community document is deleted.

Answer 2

the following code worked -
Future userRecordUpdate(DocumentReference community) async {
final instance = FirebaseFirestore.instance;
final batch = instance.batch();
var collection = instance.collection('users');
var snapshots =
await collection.where("joinedCommunity", arrayContains:
community).get();
for (var doc in snapshots.docs) {
batch.update(doc.reference, {
"joinedCommunity": FieldValue.arrayRemove([community])
});
}
await batch.commit();
}