$currentDate

Definition定义

$currentDate

The $currentDate operator sets the value of a field to the current date, either as a Date or a timestamp. $currentDate运算符将字段的值设置为当前日期，可以是Date或时间戳。The default type is Date.默认类型是Date。

The $currentDate operator has the form:$currentDate运算符的格式如下：

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

<typeSpecification> can be either:可以是：

• a boolean true to set the field value to the current date as a Date, or布尔值true，用于将字段值设置为当前日期作为日期，或
• a document { $type: "timestamp" } or { $type: "date" } which explicitly specifies the type. 文档{ $type: "timestamp" }或{ $type: "date" }明确指定类型。The operator is case-sensitive and accepts only the lowercase "timestamp" or the lowercase "date".运算符区分大小写，只接受小写"timestamp"或小写"date"。

To specify a <field> in an embedded document or in an array, use dot notation.要在嵌入文档或数组中指定<field>，请使用点表示法。

Behavior行为

Starting in MongoDB 5.0, update operators process document fields with string-based names in lexicographic order. 从MongoDB 5.0开始，更新运算符以词典顺序处理具有基于字符串的名称的文档字段。Fields with numeric names are processed in numeric order. 具有数字名称的字段按数字顺序处理。See Update Operators Behavior for details.有关详细信息，请参阅更新运算符行为。

If the field does not exist, $currentDate adds the field to a document.如果该字段不存在，$currentDate将该字段添加到文档中。

Starting in MongoDB 5.0, mongod no longer raises an error when you use an update operator like $currentDate with an empty operand expression ( { } ). 从MongoDB 5.0开始，当使用带有空操作数表达式（{ }）的更新运算符（如$currentDate）时，mongod不再引发错误。An empty update results in no changes and no oplog entry is created (meaning that the operation is a no-op).空更新不会导致任何更改，也不会创建oplog条目（这意味着该操作是无操作）。

Example示例

Create a sample collection customers with the following document:使用以下文档创建样本集合customers：

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

The following operation updates the lastModified field to the current date, the "cancellation.date" field to the current timestamp as well as updating the status field to "D" and the "cancellation.reason" to "user request".以下操作将lastModified字段更新为当前日期，"cancellation.date"字段更新为目前时间戳，并将状态字段更新为"D"，将"cancellation.reason"更新为"user request"。

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

After the operation, you can query the collection to verify the update:操作完成后，您可以查询集合以验证更新：

db.customers.find()

The updated document would resemble:更新后的文件类似于：

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

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"
   }
}