$not (aggregation)（聚合）

Definition定义

$not

Evaluates a boolean and returns the opposite boolean value; i.e. when passed an expression that evaluates to true, $not returns false; when passed an expression that evaluates to false, $not returns true.计算一个布尔值并返回相反的布尔值；即，当传递一个计算结果为true的表达式时，$not返回false；当传递一个计算结果为false的表达式时，$not返回true。

$not has the following syntax:

{ $not: [ <expression> ] }

For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。

Behavior行为

In addition to the false boolean value, $not evaluates as false the following: null, 0, and undefined values. 除了false布尔值外，$not还将以下值计算为false：null、0和undefined值。The $not evaluates all other values as true, including non-zero numeric values and arrays.$not将所有其他值（包括非零数值和数组）计算为true。

Example示例

Consider an inventory collection with the following documents:考虑使用以下文件进行inventory集合：

db.inventory.insertMany( [
   { _id: 1, item: "abc1", description: "product 1", qty: 300 },
   { _id: 2, item: "abc2", description: "product 2", qty: 200 },
   { _id: 3, item: "xyz1", description: "product 3", qty: 250 },
   { _id: 4, item: "VWZ1", description: "product 4", qty: 300 },
   { _id: 5, item: "VWZ2", description: "product 5", qty: 180 }
] )

The following operation uses the $not operator to determine if qty is not greater than 250:以下操作使用$not运算符来确定qty是否不大于250：

db.inventory.aggregate(
   [
     {
       $project:
          {
            item: 1,
            result: { $not: [ { $gt: [ "$qty", 250 ] } ] }
          }
     }
   ]
)

The operation returns the following results:该操作返回以下结果：

{ _id: 1, item: "abc1", result: false }
{ _id: 2, item: "abc2", result: true }
{ _id: 3, item: "xyz1", result: true }
{ _id: 4, item: "VWZ1", result: false }
{ _id: 5, item: "VWZ2", result: true }