[{"name":"admin","datasets":[{"name":"test","datapoints":[1,6,4,3,8,5,3],"_id":{"$oid":"619288f16733758444a28728"}},{"name":"more datasets","datapoints":[1,2,3,4,5,6,7,8],"_id":{"$oid":"619289086733758444a2872a"}}]}]

db.collection.find({name:"admin","datasets.name":"test"},{"datasets.$.datapoints":{$slice:[0,3]}})