$not (aggregation)

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将返回true。

$not has the following syntax:具有以下语法：

{ $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, $not evaluates as false the following: null, 0, and undefined values. 除了false布尔值之外，$not还将以下值计算为false：null、0和undefined的值。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实例

Consider an inventory collection with the following documents:考虑一个包含以下文档的inventory集合：

{ "_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 }