db.cities.aggregate([ { "$project": { "id_city": { "$toString": "$_id" } } }, //combiente a string los ObjectIds { "$lookup": { "localField": "id_city", "from": "itineraries", "foreignField": "cityID", as: "matching_docs" } }, { $match: { matching_docs: { $ne: [] } // Filtrar documentos que tienen coincidencias } } ])