$allElementsTrue (aggregation)¶

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Definition¶

$allElementsTrue¶

New in version 2.6.

Evaluates an array as a set and returns true if no element in the array is false. Otherwise, returns false. An empty array returns true.

$allElementsTrue has the following syntax:

{ $allElementsTrue: [ <expression> ] }

The <expression> itself must resolve to an array, separate from the outer array that denotes the argument list. For more information on expressions, see Expressions.

Behavior¶

If a set contains a nested array element, $allElementsTrue does not descend into the nested array but evaluates the array at top-level.

In addition to the false boolean value, $allElementsTrue evaluates as false the following: null, 0, and undefined values. The $allElementsTrue evaluates all other values as true, including non-zero numeric values and arrays.

Example		Result
`{ $allElementsTrue: [ [ true, 1, "someString" ] ] }`		`true`
`{ $allElementsTrue: [ [ [ false ] ] ] }`		`true`
`{ $allElementsTrue: [ [ ] ] }`		`true`
`{ $allElementsTrue: [ [ null, false, 0 ] ] }`		`false`

Example¶

Consider an survey collection with the following documents:

{ "_id" : 1, "responses" : [ true ] }
{ "_id" : 2, "responses" : [ true, false ] }
{ "_id" : 3, "responses" : [ ] }
{ "_id" : 4, "responses" : [ 1, true, "seven" ] }
{ "_id" : 5, "responses" : [ 0 ] }
{ "_id" : 6, "responses" : [ [ ] ] }
{ "_id" : 7, "responses" : [ [ 0 ] ] }
{ "_id" : 8, "responses" : [ [ false ] ] }
{ "_id" : 9, "responses" : [ null ] }
{ "_id" : 10, "responses" : [ undefined ] }

The following operation uses the $allElementsTrue operator to determine if the responses array only contains values that evaluate to true:

db.survey.aggregate(
   [
     { $project: { responses: 1, isAllTrue: { $allElementsTrue: [ "$responses" ] }, _id: 0 } }
   ]
)

The operation returns the following results:

{ "responses" : [ true ], "isAllTrue" : true }
{ "responses" : [ true, false ], "isAllTrue" : false }
{ "responses" : [ ], "isAllTrue" : true }
{ "responses" : [ 1, true, "seven" ], "isAllTrue" : true }
{ "responses" : [ 0 ], "isAllTrue" : false }
{ "responses" : [ [ ] ], "isAllTrue" : true }
{ "responses" : [ [ 0 ] ], "isAllTrue" : true }
{ "responses" : [ [ false ] ], "isAllTrue" : true }
{ "responses" : [ null ], "isAllTrue" : false }
{ "responses" : [ null ], "isAllTrue" : false }

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