$in (aggregation)¶

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Definition
Behavior
Example

Definition¶

$in¶

3.4 新版功能.

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

$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¶

A collection named fruit has the following documents:

{ "_id" : 1, "location" : "24th Street",
  "in_stock" : [ "apples", "oranges", "bananas" ] }
{ "_id" : 2, "location" : "36th Street",
  "in_stock" : [ "bananas", "pears", "grapes" ] }
{ "_id" : 3, "location" : "82nd Street",
  "in_stock" : [ "cantaloupes", "watermelons", "apples" ] }

The following aggregation operation looks at the in_stock array in each document and determines whether the string bananas is present.

db.fruit.aggregate([
  {
    $project: {
      "store location" : "$location",
      "has bananas" : {
        $in: [ "bananas", "$in_stock" ]
      }
    }
  }
])

The operation returns the following results:

{ "_id" : 1, "store location" : "24th Street", "has bananas" : true }
{ "_id" : 2, "store location" : "36th Street", "has bananas" : true }
{ "_id" : 3, "store location" : "82nd Street", "has bananas" : false }

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