$currentDate（更新操作符）

定义

$currentDate

The $currentDate operator sets the value of a field to the current date, either as a Date or a timestamp. The default type is Date.

兼容性

可以使用 $currentDate 查找托管在以下环境中的部署：

• MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

• MongoDB Enterprise：基于订阅、自我管理的 MongoDB 版本

• MongoDB Community：源代码可用、免费使用且可自行管理的 MongoDB 版本

语法

The $currentDate operator has the form:

{ $currentDate: { <field1>: <typeSpecification1>, ... } }

<typeSpecification> 可以为以下任一项：

• 布尔值为 true 可将字段值设置为当前日期作为“日期”，或者

• 显式指定类型的文档 { $type: "timestamp" } 或 { $type: "date" }。该操作符区分大小写，仅接受小写的 "timestamp" 或小写的 "date"。

要在嵌入式文档或数组中指定 <field>，请使用点符号。

行为

从 MongoDB 5.0 开始，更新操作符按字典顺序处理具有基于字符串的名称的文档字段。具有数字名称的字段按数字顺序处理。详情请参阅更新操作符行为。

$currentDate sets the specified field to the date when $currentDate was run.

If the field does not exist, $currentDate adds the field to a document.

Starting in MongoDB 5.0, mongod no longer raises an error when you use an update operator like $currentDate with an empty operand expression ( { } ). An empty update results in no changes and no oplog entry is created (meaning that the operation is a no-op).

例子

创建一个包含以下文档的样本集合 customers：

db.customers.insertOne(
   { _id: 1, status: "a", lastModified: ISODate("2013-10-02T01:11:18.965Z") }
)

以下操作将 lastModified 字段更新为当前日期，将 "cancellation.date" 字段更新为当前时间戳，并将 status 字段更新为 "D" 以及 "cancellation.reason" 至 "user request"。

db.customers.updateOne(
   { _id: 1 },
   {
     $currentDate: {
        lastModified: true,
        "cancellation.date": { $type: "timestamp" }
     },
     $set: {
        "cancellation.reason": "user request",
        status: "D"
     }
   }
)

$currentDate sets the specified field to the date when $currentDate was run.

要验证更新：

db.customers.find()

更新后的文档类似于：

{
   "_id" : 1,
   "status" : "D",
   "lastModified" : ISODate("2020-01-22T21:21:41.052Z"),
   "cancellation" : {
      "date" : Timestamp(1579728101, 1),
      "reason" : "user request"
   }
}

The lastModified field is set to the date when $currentDate was run in the update example shown earlier.

db.customers.updateOne(
  { _id: 1 },
  [
   { $set: { lastModified: "$$NOW", cancellation: {date: "$$CLUSTER_TIME", reason: "user request"}, status: "D" } }
  ]
)

db.customers.find().pretty()

{
   "_id" : 1,
   "status" : "D",
   "lastModified" : ISODate("2020-01-22T21:02:18.994Z"),
   "cancellation" : {
      "date" : Timestamp(1579726934, 2),
      "reason" : "user request"
   }
}