$anyElementTrue (expression operator)

Definition

$anyElementTrue

Evaluates an array as a set and returns true if any of the elements are true and false otherwise. An empty array returns false.

$anyElementTrue has the following syntax:

{ $anyElementTrue: [ <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, $anyElementTrue does not descend into the nested array but evaluates the array at top-level.

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

Example

Create an example collection named survey with the following documents:

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 $anyElementTrue operator to determine if the responses array contains any value that evaluates to true:

db.survey.aggregate(
  [
    { $project: { responses: 1, isAnyTrue: { $anyElementTrue: [ "$responses" ] }, _id: 1 } }
  ]
)

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

In the results:

• Document with _id: 1 is true because the element inside the responses array evaluates as true.
• Documents with _id: 2 and _id: 4 are true because at least one element inside the responses array evaluates as true.

• Documents with _id: 6, _id: 7, and _id: 8 are true because the responses array, which is the array that $anyElementTrue evaluated for the operation, contains a nested array, which $anyElementTrue always evaluates as true.