db={positions:[{"_id":"63d78e5096109914dc963431","field":"electrical","regularName":"elc",},{"_id":"63d78e5096109914dc963433","field":"mechanic","regularName":"mec",}],doc:[{name:"projectName1",positions:[{position_id:"63d78e5096109914dc963431",use:true,wage:0,default:true},{position_id:"63d78e5096109914dc963433",use:true,wage:0,default:true}]}]}

db.doc.aggregate([{$lookup:{from:"positions",localField:"positions.position_id",foreignField:"_id",as:"positionDetails"}},{$set:{positions:{$map:{input:"$positions",as:"pos",in:{$mergeObjects:["$$pos",{"positionDetails":{$first:{$filter:{input:"$positionDetails",cond:{$eq:["$$pos.position_id","$$this._id"]}}}}}]}}}}},{$unset:"positionDetails"}])