$toDate (expression operator)（表达式运算符）

Definition定义

$toDate

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

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

{
   $toDate: <expression>
}

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

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

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

Tip

• $convert
• $dateFromString

Behavior行为

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

The following table lists some conversion to date examples:下表列出了一些迄今为止的转换示例：

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, price: 2, order_date: new Date( "2018-03-20" ) },
   { _id: 2, item: "pie", qty: 10, price: 3, order_date: new Date( "2018-03-22" ) },
   { _id: 3, item: "ice cream", qty: 2, price: 4, order_date: "2018-03-15" },
   { _id: 4, item: "almonds" , qty: 5, price: 7, order_date: "2018-03-15 +10:00" }
] )

The following aggregation operation on the orders collection converts the order_date to date before sorting by the date value:以下对orders集合的聚合操作将order_date转换为日期，然后再按日期值排序：

// Define stage to add convertedDate field with the converted order_date value定义阶段以添加具有转换后的order_date值的convertedDate字段

dateConversionStage = {
   $addFields: {
      convertedDate: { $toDate: "$order_date" }
   }
};

// Define stage to sort documents by the converted date定义按转换日期对文档进行排序的阶段

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

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

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

{
   _id: 4,
   item: 'almonds',
   qty: 5,
   price: 7,
   order_date: '2018-03-15 +10:00',
   convertedDate: ISODate("2018-03-14T14:00:00.000Z")
},
{
   _id: 3,
   item: 'ice cream',
   qty: 2,
   price: 4,
   order_date: '2018-03-15',
   convertedDate: ISODate("2018-03-15T00:00:00.000Z")
},
{
   _id: 1,
   item: 'apple',
   qty: 5,
   price: 2,
   order_date: ISODate("2018-03-20T00:00:00.000Z"),
   convertedDate: ISODate("2018-03-20T00:00:00.000Z")
},
{
   _id: 2,
   item: 'pie',
   qty: 10,
   price: 3,
   order_date: ISODate("2018-03-22T00:00:00.000Z"),
   convertedDate: ISODate("2018-03-22T00:00:00.000Z")
}