[{_id:ObjectId("62c3e31931e7df585c39e4e1"),activity_id:ObjectId("62c3e31931e7df585c39e4df"),challenge_id:ObjectId("62bd543c3a3937000958f2dd"),status:"active",createdAt:ISODate("2022-07-05T07:07:05.823Z"),updatedAt:ISODate("2022-07-05T07:07:05.823Z")},{_id:ObjectId("62c3e33f299750585cc70b23"),activity_id:ObjectId("62c3e33e299750585cc70b21"),challenge_id:ObjectId("62bd543c3a3937000958f2dd"),status:"active",createdAt:ISODate("2022-07-05T07:07:43.612Z"),updatedAt:ISODate("2022-07-05T07:07:43.612Z")},{_id:ObjectId("62c3e359341e86585c65c714"),activity_id:ObjectId("62c3e359341e86585c65c712"),challenge_id:ObjectId("62bd543c3a3937000958f2dd"),status:"active",createdAt:ISODate("2022-07-05T07:08:09.409Z"),updatedAt:ISODate("2022-07-05T07:08:09.409Z")}]

db.collection.aggregate([{$group:{_id:0,res:{$push:{$toString:"$challenge_id"}}}},{$project:{res:1,_id:0}}])