db={"User":[{"_id":1,"homeCountry":"USA"},{"_id":2,"homeCountry":"SL"}],"traveladds":[{"_id":1,"k2":"value","creatorId":1}]}

db.User.aggregate([{$lookup:{from:"traveladds",let:{personalId:"$_id"},pipeline:[{$match:{$expr:{$eq:["$creatorId","$$personalId"]}}}],as:"stockdata"}},])