$toObjectId (aggregation)

Definition定义

$toObjectId

Converts a value to an ObjectId(). 将值转换为ObjectId()。If the value cannot be converted to an ObjectId, $toObjectId errors. 如果无法将值转换为ObjectId，则$toObjectId将出错。If the value is null or missing, $toObjectId returns null.如果该值为null或缺失，$toObjectId将返回null。

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

{
   $toObjectId: <expression>
}

The $toObjectId takes any valid expression.$toObjectId采用任何有效的表达式。

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

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

Tip

See also: 另请参阅：

$convert

Behavior行为

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

The following table lists some conversion to date examples:下表列出了一些转换到日期的示例：

Example实例

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

db.orders.insertMany( [
   { _id: "5ab9cbe531c2ab715d42129a", item: "apple", qty: 10 },
   { _id: ObjectId("5ab9d0b831c2ab715d4212a8"), item: "pie", qty: 5 },
   { _id: ObjectId("5ab9d2d331c2ab715d4212b3"), item: "ice cream", qty: 20 },
   { _id: "5ab9e16431c2ab715d4212b4", item: "almonds", qty: 50 },
] )

The following aggregation operation on the orders collection converts the _id to ObjectId before sorting by the value:以下对orders集合的聚合操作将_id转换为ObjectId，然后再按值排序：

// Define stage to add convertedId field with converted _id value

idConversionStage = {
   $addFields: {
      convertedId: { $toObjectId: "$_id" }
   }
};

// Define stage to sort documents by the converted qty values

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


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

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

{
  _id: '5ab9e16431c2ab715d4212b4',
  item: 'almonds',
  qty: 50,
  convertedId: ObjectId("5ab9e16431c2ab715d4212b4")
},
{
  _id: ObjectId("5ab9d2d331c2ab715d4212b3"),
  item: 'ice cream',
  qty: 20,
  convertedId: ObjectId("5ab9d2d331c2ab715d4212b3")
},
{
  _id: ObjectId("5ab9d0b831c2ab715d4212a8"),
  item: 'pie',
  qty: 5,
  convertedId: ObjectId("5ab9d0b831c2ab715d4212a8")
},
{
  _id: '5ab9cbe531c2ab715d42129a',
  item: 'apple',
  qty: 10,
  convertedId: ObjectId("5ab9cbe531c2ab715d42129a")
}