db={"products":[{"_id":{"$oid":"62414fc1d2564b4554120234"},"amountIsFixed":false,"amount":10,"active":false,"user_id":"62414fc1d24545456d4957ec"}],"users":[{"_id":{"$oid":"62414fc1d24545456d4957ec"},"fullname":"Test"}]}

db.products.aggregate([{$set:{user_ob_id:{$toObjectId:"$user_id"}}},{$lookup:{from:"users",localField:"user_ob_id",foreignField:"_id",as:"userDetails"}},{$unwind:"$userDetails"},{$project:{fullname:"$userDetails.fullname",id:"$userDetails.id",amount:1}}])