Database
[{comment_id:1,content_id:1,comment_created:"2020-12-10T09:21:48.861Z"},{comment_id:2,content_id:1,comment_created:"2020-12-12T09:21:48.861Z"},{comment_id:3,content_id:2,comment_created:"2020-12-15T09:21:48.861Z"},{comment_id:4,content_id:2,comment_created:"2020-12-10T09:21:48.861Z"}]
Query
db.collection.aggregate([{$match:{content_id:{$in:[1,2]}}},{$sort:{comment_created:-1}},{$group:{_id:"$content_id",recentComment:{$first:"$$ROOT"}}},{$replaceRoot:{newRoot:"$recentComment"}}])