[{"collection":"posts","count":50,"content":{"_id":{"type":"autoincrement","autoType":"int"},"name":{"type":"stringFromParts","parts":[{"type":"constant","constVal":"POST "},{"type":"int","minInt":1,"maxInt":10}]}}}]

db.posts.aggregate([{"$sortByCount":"$name"},{"$facet":{"maxCount":[{"$limit":1},{"$project":{"_id":0,"count":1}}],"postsWithCount":[{"$project":{"_id":0,"name":"$_id","count":1}}]}},{"$project":{"postsWithMaxCount":{"$filter":{"input":"$postsWithCount","cond":{"$eq":["$$this.count",{"$first":"$maxCount.count"}]}}}}},{"$unwind":"$postsWithMaxCount"},{"$replaceWith":"$postsWithMaxCount"}])