db={"country":[{"_id":"5fbc7cc705253c2da4820425","name":"USA"},{"_id":"5fbc7cc705253c2da4820422","name":"PERU"},{"_id":"5fbc7cc705253c2da4820423","name":"COLOMBIA"}],"travel_reservation":[{"name_person":"pablo rojas","countrys_id":[{"country_id":"5fbc7cc705253c2da4820425"},{"country_id":"5fbc7cc705253c2da4820423"}]},{"name_person":"Anuel","countrys_id":[{"country_id":"5fbc7cc705253c2da4820422"}]},{"name_person":"Laura","countrys_id":[]}]}}

db.travel_reservation.aggregate([{$unwind:{path:"$countrys_id",preserveNullAndEmptyArrays:true}},{$lookup:{from:"country",let:{ci:"$countrys_id.country_id"},pipeline:[{$match:{$expr:{$eq:["$_id","$$ci"]}}}],as:"join",}},{$addFields:{join:{$arrayElemAt:["$join",0]}}},{$group:{_id:"$_id",countrys:{$push:"$join"},name_person:{$first:"$name_person"}}}])