Firestore order by and limit

Question

I am writing a simple chat feature within my application. Given the fact some users will have multiple messages within a conversation, I need to provide reverse pagination so they can load previous messages when scrolling. From the model side, each message has a timestamp (a time interval) so that it's easy to query.

I'm able to query and order desc from the date field, however... the second I limit the response to X number of items.. somehow the date field is omitted and I get the first 5 in the collection, not the 5 I want ordered by date. How can I fix this?

Example

message one: 1

message two: 2

message three: 3

message four: 4

When I order desc, I get the right list [1,2,3,4,5]. When I limit(2), it gives me [1,2] but what I really want is [3, 4].

I am missing something obvious?

Update:

collection: "conversations". sub collection "messages".

Collection id is done by aggregating both user ids so it's easier to query a conversation of 2 users.

db.collection("conversations")
                .document(identifier)
                .collection("messages")
                .order(by: "sentDate", descending: false)
                .limit(to: 5)
                .addSnapshotListener({ snapshot, error in
                    guard let document = snapshot else { return }
                    let messages = document.documents.map {
                        return Message.fromJSON($0.data())
                    }

                    observable.onNext(messages as! [Message])
                    observable.onCompleted()
                })

Answer 1

If you are getting [1,2,3,4] during .order(by: "sentDate", descending: false) ,

and [1,2] when using .order(by: "sentDate", descending: false).limit(to: 2) , then you can do the following:

1. Use .order(by: "sentDate", descending: true) to get [4,3,2,1]
2. Then use .limit(to: 2) to get [4,3]
3. At last use .reverse() before mapping it.

(ie. let messages = document.documents.reverse().map line in your code)

And you will get [3,4].