$in (expression operator)

Definition

$in

Returns a boolean indicating whether a specified value is in an array.

Note

This document describes the $in aggregation operator. For the $in query operator, see $in (query predicate operator).

$in has the following operator expression syntax:

{ $in: [ <expression>, <array expression> ] }

Operand	Description
<expression>	Any valid expression expression.
<array expression>	Any valid expression that resolves to an array.

Unlike the $in query operator, the aggregation $in operator does not support matching by regular expressions.

Example	Results
{ $in: [ 2, [ 1, 2, 3 ] ] }	true
{ $in: [ "abc", [ "xyz", "abc" ] ] }	true
{ $in: [ "xy", [ "xyz", "abc" ] ] }	false
{ $in: [ [ "a" ], [ "a" ] ] }	false
{ $in: [ [ "a" ], [ [ "a" ] ] ] }	true
{ $in: [ /^a/, [ "a" ] ] }	false
{ $in: [ /^a/, [ /^a/ ] ] }	true

Behavior

$in fails with an error in either of the following cases: if the $in expression is not given exactly two arguments, or if the second argument does not resolve to an array.

Example

The following example uses the movies collection from the sample_mflix sample dataset. For details on how to load this dataset into your deployment, see Load the sample dataset.

This aggregation operation looks at the genres array in each movie and determines whether the string "Drama" is present:

db.movies.aggregate( [
   { $match: { runtime: { $gt: 1000 } } },
   {
      $project:
        {
          _id: 0,
          title: 1,
          hasDrama: { $in: [ "Drama", "$genres" ] }
        }
   }
] )

[
  {
    title: 'Centennial',
    hasDrama: true
  },
  {
    title: 'Baseball',
    hasDrama: false
  }
]