$allElementsTrue (aggregation)

Definition

• $allElementsTrue

New in version 2.6.

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

$allElementsTrue has the following syntax:

{ $allElementsTrue: [ <expression> ] }

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

Behavior

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

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

Example

Create an example collection named survey with the followingdocuments:

db.survey.insertMany([
   { "_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 $allElementsTrueoperator to determine if the responses array only contains valuesthat 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" : [ undefined ], "isAllTrue" : false }