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- $allElementsTrue (aggregation)
$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 }