$toString (aggregation)¶

Definition定义¶

$toString¶

New in version 4.0.版本4.0中的新功能。

Converts a value to a string. If the value cannot be converted to a string, $toString errors. 将值转换为字符串。如果无法将值转换为字符串，$toString错误。If the value is null or missing, $toString returns null.如果值为null或缺少，$toString返回null。

$toString has the following syntax:语法如下所示：

{
   $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表达式的缩写：

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

See also参阅

$convert and 和$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.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" },
])

The following aggregation operation on the orders collection converts the zipcode to string before sorting by the string value:在按字符串值排序之前，orders集合上的以下聚合操作会将zipcode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:该操作将返回以下文档：

{ "_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" }