[{"reli":[{"_id":"abcdef","count":6},{"_id":"ghij","count":1},],"prov":[{"_id":"hello","count":63},{"_id":"hey","count":9},]}]

db.collection.aggregate([{"$project":{"reli":{"$map":{"input":"$reli","in":{"$arrayToObject":[[{k:"$$this._id",v:"$$this.count"}]]}}},"prov":{"$map":{"input":"$prov","in":{"$arrayToObject":[[{k:"$$this._id",v:"$$this.count"}]]}}}}}])