db={"req":[{reqId:"A123",status:"1",location:"hyd"}],"reqUser":[{req:"A123",userId:"U1787",designation:"employee"}]}

db.req.aggregate([{"$lookup":{"from":"reqUser","localField":"reqId","foreignField":"req","as":"user"}},{$set:{userId:{$arrayElemAt:["$user.userId",0]}}},{"$project":{user:0}}])