$ toString（聚合）

db.orders.insert( [
   { _id: 1, item: "apple",  qty: 5, zipcode: 12345 },
   { _id: 2, item: "pie",  qty: 10, zipcode: 11111 },
   { _id: 3, item: "ice cream",  zipcode: "12345" },
   { _id: 4, item: "almonds", qty: 2, zipcode: "12345-0030" },
])

// Define stage to add convertedZipCode field with the converted zipcode value

zipConversionStage = {
   $addFields: {
      convertedZipCode: { $toString: "$zipcode" }
   }
};

// Define stage to sort documents by the converted zipcode

sortStage = {
   $sort: { "convertedZipCode": 1 }
};

db.orders.aggregate( [
  zipConversionStage,
  sortStage
])

{ "_id" : 2, "item" : "pie", "qty" : 10, "zipcode" : 11111, "convertedZipCode" : "11111" }
{ "_id" : 1, "item" : "apple", "qty" : 5, "zipcode" : 12345, "convertedZipCode" : "12345" }
{ "_id" : 3, "item" : "ice cream", "zipcode" : "12345", "convertedZipCode" : "12345" }
{ "_id" : 4, "item" : "almonds", "qty" : 2, "zipcode" : "12345-0030", "convertedZipCode" : "12345-0030" }