$toLong (aggregation)（聚合）

Definition定义

$toLong

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

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

{
   $toLong: <expression>
}

The $toLong takes any valid expression.$toLong接受任何有效表达式。

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

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

Tip

$convert

Behavior行为

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

The following table lists some conversion to long examples:下表列出了一些转换为长示例：

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: Int32(5) },
   { _id: 2, item: "pie", qty: "100" },
   { _id: 3, item: "ice cream", qty: Long("500") },
   { _id: 4, item: "almonds", qty: "50" },
] )

The following aggregation operation on the orders collection converts the qty to long before sorting by the value:orders集合上的以下聚合操作将qty转换为按值排序之前的很久：

// Define stage to add convertedQty field with converted qty value定义阶段以添加具有转换数量值的转换数量字段

qtyConversionStage = {
   $addFields: {
      convertedQty: { $toLong: "$qty" }
   }
};

// Define stage to sort documents by the converted qty values定义按转换后的数量值对文档进行排序的阶段

sortStage = {
   $sort: { "convertedQty": -1 }
};


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

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

{ _id: 3, item: 'ice cream', qty: Long("500"), convertedQty: Long("500") },
{ _id: 2, item: 'pie', qty: '100', convertedQty: Long("100") },
{ _id: 4, item: 'almonds', qty: '50', convertedQty: Long("50") },
{ _id: 1, item: 'apple', qty: 5, convertedQty: Long("5") }