For the furure coders, do not follow the advice of using perl.
Think about the following.
You want the n-most recent comments as clearly indicated in the requirements. Let say you have 10000 comments. Then every time somebody use your app you will download 10000 comments from the server just to get rid of most of them. Better have the server do it. In addition, if you have an index on the date the server does not even have to fetch the comments from storage to do the sort and limit required to get the n-most recent.
And do not forget that you are here to learn mongo. You are not here for a grade. If it take a few hours to understand a concept, well that’s the price to pay.