db={"users":[{_id:"1",userId:1,userName:"User1",OrderIds:[{oId:1},{oId:3}]},{_id:"2",userId:2,userName:"User2",OrderIds:[{oId:2},{oId:3}]}],"orders":[{_id:"1",orderId:1,orderName:"Pen"},{_id:"2",orderId:2,orderName:"Book"},{_id:"3",orderId:3,orderName:"Chair"}]}

db.users.aggregate([{"$lookup":{"from":"orders","localField":"OrderIds.oId","foreignField":"orderId","as":"OrderIds"}}])