$max

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Definition定义

$max

The $max operator updates the value of the field to a specified value if the specified value is greater than the current value of the field. 如果指定值大于字段的当前值，$max运算符将字段的值更新为指定值。The $max operator can compare values of different types, using the BSON comparison order.$max运算符可以使用BSON比较顺序比较不同类型的值。

The $max operator expression has the form:$max运算符表达式的形式如下：

{ $max: { <field1>: <value1>, ... } }

To specify a <field> in an embedded document or in an array, use dot notation.若要在嵌入式文档或数组中指定一个<field>，请使用点表示法。

Behavior行为

If the field does not exists, the $max operator sets the field to the specified value.如果该字段不存在，$max运算符将该字段设置为指定值。

Examples示例

Use $max to Compare Numbers使用$max来比较数字

Consider the following document in the collection scores:在集合scores中考虑以下文档：

{ _id: 1, highScore: 800, lowScore: 200 }

The highScore for the document currently has the value 800. 文档的highScore当前为800。The following operation uses $max to compare the 800 and the specified value 950 and updates the value of highScore to 950 since 950 is greater than 800:以下操作使用$max比较800和指定值950，并将highScore的值更新为950，因为950大于800：

db.scores.update( { _id: 1 }, { $max: { highScore: 950 } } )

The scores collection now contains the following modified document:scores集合现在包含以下修改过的文档：

{ _id: 1, highScore: 950, lowScore: 200 }

The next operation has no effect since the current value of the field highScore, i.e. 950, is greater than 870:由于字段highScore（即950）的当前值大于870，因此下一个操作无效：

db.scores.update( { _id: 1 }, { $max: { highScore: 870 } } )

The document remains unchanged in the scores collection:该文档在scores集合中保持不变：

{ _id: 1, highScore: 950, lowScore: 200 }

Use $max to Compare Dates使用$max来比较日期

Consider the following document in the collection tags:在集合tags中考虑以下文档：

{
  _id: 1,
  desc: "crafts",
  dateEntered: ISODate("2013-10-01T05:00:00Z"),
  dateExpired: ISODate("2013-10-01T16:38:16.163Z")
}

The following operation compares the current value of the dateExpired field, i.e. ISODate("2013-10-01T16:38:16.163Z"), with the specified date new Date("2013-09-30") to determine whether to update the field:以下操作将dateExpired字段的当前值（即ISODate("2013-10-01T16:38:16.163Z")）与指定的日期new Date("2013-09-30")进行比较，以确定是否更新该字段：

db.tags.update(
   { _id: 1 },
   { $max: { dateExpired: new Date("2013-09-30") } }
)

The operation does not update the dateExpired field:该操作不会更新dateExpired字段：

{
   _id: 1,
   desc: "decorative arts",
   dateEntered: ISODate("2013-10-01T05:00:00Z"),
dateExpired: ISODate("2013-10-01T16:38:16.163Z")}

See also参阅

db.collection.update(), db.collection.findAndModify()