Firestore query to check if an id is in a collection and then going to the subcollection in a single query

I have a firestore with 2 root collections, properties and users. Each of these properties has a list of "huurders" which are just userId's, and a subcollection of rooms each of which also has a "huurder" field which contains a UserId.

I'm trying to get a list of all rooms where the "y" field is equal to a given id. Right now I'm just checking if "y" contains the UserId i'm looking for, but I'm struggling about how to go into the subcollection x afterwards.

This is my function

override fun get(userId: String): Flow<RoomsResponse> = callbackFlow {
    val snapshotListener = dbRef.collection("properties").whereArrayContains("", ).addSnapshotListener() { snapshot, e ->
        val roomsResponse = if (snapshot != null) {
            val rooms = snapshot.toObjects(Room::class.java)
            Response.Success(rooms)
        } else {
            Response.Failure(e)
        }
        trySend(
    awaitClose { snapshotListener.remove()
}
}

This is incorrect given this would just return me a list of properties where the is in the array . I need to go one layer deeper and get the rooms. My firestore will be linked as an image

Firestore queries always returns the documents that you query on. They can't return documents from nested collection, or otherwise linked data.

So if you want to get a list rooms from the database, you'll need to query the rooms subcollection. To query across all collections named rooms, you can use a collection group query. And since you seem to have a huurder field in each rooms document, you should be able to get the correct results with:

dbRef.collectionGroup("rooms").whereEqualTo("huurder", userId)

This will give you a list of rooms document snapshots.

If you then want to find the property of a specific DocumentSnapshot in these results, you can get its DocumentReference and then check the parent of its parent.