$toString (aggregation)

Definition定义

$toString

Converts a value to a string. If the value cannot be converted to a string, $toString errors. If the value is null or missing, $toString returns null.将值转换为字符串。如果值无法转换为字符串，则$toString错误。如果该值为null或缺失，$toString将返回null。

$toString has the following syntax:具有以下语法：

{
   $toString: <expression>
}

The $toString takes any valid expression.$toString接受任何有效的表达式。

The $toString is a shorthand for the following $convert expression:$toString是以下$convert表达式的简写：

{ $convert: { input: <expression>, to: "string" } }

Tip

• $convert
• $dateToString

Behavior行为

The following table lists the input types that can be converted to a string:下表列出了可以转换为字符串的输入类型：

The following table lists some conversion to string examples:下表列出了一些转换为字符串的示例：

Example示例

Create a collection orders with the following documents:使用以下文档创建集合orders：

db.orders.insertMany( [
   { _id: 1, item: "apple",  qty: 5, zipcode: 93445 },
   { _id: 2, item: "almonds", qty: 2, zipcode: "12345-0030" },
   { _id: 3, item: "peaches",  qty: 5, zipcode: 12345 },
] )

The following aggregation operation on the orders collection converts the zipcode to string before sorting by the string value:orders集合上的以下聚合操作在按字符串值排序之前将zipcode（邮政编码）转换为字符串：

// 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
] )

The operation returns the following documents:该操作返回以下文档：

{
  _id: 3,
  item: 'peaches',
  qty: 5,
  zipcode: 12345,
  convertedZipCode: '12345'
},
{
  _id: 2,
  item: 'almonds',
  qty: 2,
  zipcode: '12345-0030',
  convertedZipCode: '12345-0030'
},
{
  _id: 1,
  item: 'apple',
  qty: 5,
  zipcode: 93445,
  convertedZipCode: '93445'
}