[{"name":"n1","posts":[{"title":"t1","posted_on":1},{"title":"t2","posted_on":2},{"title":"t3","posted_on":3},{"title":"t4","posted_on":4},{"title":"t5","posted_on":5}]}]

db.collection.aggregate([{"$match":{"name":{"$eq":"n1"}}},{"$set":{"posts":{"$slice":[{"$filter":{"input":"$posts","cond":{"$and":[{"$gt":["$$this.posted_on",1]},{"$lt":["$$this.posted_on",5]}]}}},2]}}}])