$min

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Definition定义

$min

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

{ $min: { <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 exist, the $min operator sets the field to the specified value.如果该字段不存在，$min运算符将该字段设置为指定值。

For comparisons between values of different types, such as a number and a null, $min uses the BSON comparison order.对于不同类型的值（如数字和空值）之间的比较，$min使用BSON比较顺序。

Examples示例

Use $min to Compare Numbers使用$min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 lowScore for the document currently has the value 200. 文档的lowScore当前的值为200。The following operation uses $min to compare 200 to the specified value 150 and updates the value of lowScore to 150 since 150 is less than 200:以下操作使用$min将200与指定值150进行比较，并将lowScore的值更新为150，因为150小于200：

db.scores.update( { _id: 1 }, { $min: { lowScore: 150 } } )

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: 800, lowScore: 150 }

The next operation has no effect since the current value of the field lowScore, i.e 150, is less than 250:由于字段lowScore（即150）的当前值小于250，因此下一个操作无效：

db.scores.update( { _id: 1 }, { $min: { lowScore: 250 } } )

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

{ _id: 1, highScore: 800, lowScore: 150 }

Use $min to Compare Dates使用$min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("2013-10-01T16:38:16Z")
}

The following operation compares the current value of the dateEntered field, i.e. ISODate("2013-10-01T05:00:00Z"), with the specified date new Date("2013-09-25") to determine whether to update the field:以下操作将dateEntered字段的当前值（即ISODate("2013-10-01T05:00:00Z")与指定的日期new Date("2013-09-25")进行比较，以确定是否更新该字段：

db.tags.update(
   { _id: 1 },
   { $min: { dateEntered: new Date("2013-09-25") } }
)

The operation updates the dateEntered field:该操作将更新dateEntered字段：

{
  _id: 1,
  desc: "crafts",
dateEntered: ISODate("2013-09-25T00:00:00Z"),  dateExpired: ISODate("2013-10-01T16:38:16Z")
}

See also参阅

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