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$not (aggregation)

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$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.

$not has the following syntax:

{ $not: [ <expression> ] }

For more information on expressions, see Expressions.

In addition to the false boolean value, $not evaluates as false the following: null, 0, and undefined values. The $not evaluates all other values as true, including non-zero numeric values and arrays.

Example
Result
{ $not: [ true ] }
false
{ $not: [ [ false ] ] }
false
{ $not: [ false ] }
true
{ $not: [ null ] }
true
{ $not: [ 0 ] }
true

Consider an inventory collection with the following documents:

{ "_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:

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

The operation returns the following results:

{ "_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 }
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