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
    }
  }
])