db={"country":[{"_id":"5fbc7cc705253c2da4820425","country":"USA"}],"city":[{"_id":"5fbc7cc705253c2da482025f","city":"New York","country_id":"5fbc7cc705253c2da4820425"}],"travel_reservation":[{"name_person":"pablo rojas","city_id":"5fbc7cc705253c2da482025f"}]}}

db.travel_reservation.aggregate([{$lookup:{from:"city",localField:"city_id",foreignField:"_id",as:"city",}},{$project:{city:1,name:1,_id:1},},])