MongoDB $arrayElemAt Operator

MongoDB provides different types of array expression operators that are used in the aggregation pipeline stages and $arrayElemAt operator is one of them. This operator is used to return the element present on the specified index of the given array.

Syntax:

{ $arrayElemAt: [ <array>, <index> ] }

Here, the array must be a valid expression until it resolves to an array and index must be a valid expression until it resolves to an integer.

If the value of index is positive, then this operator will return the element at the index position, from the start of the array.
If the value of index is negative, then this operator will return the element at the index position, from the end of the array.
If the index exceeds the array bounds, then this operator does not return any result.

Examples:

In the following examples, we are working with:

Database: GeeksforGeeks

Collection: arrayExample

Document: three documents that contain the details in the form of field-value pairs.

Using $arrayElemAt Operator:

In this example, we are going to find the elements of the array(i.e., the value of fruits field) on the specified index using $arrayElemAt operator.

db.arrayExample.aggregate([
... {$match: {name: "Bongo"}},
... {$project: {
... firstItem: {$arrayElemAt: ["$fruits", 0]},
... lastItem: {$arrayElemAt: ["$fruits", -1]}}}])

Using $arrayElemAt Operator in the Embedded Document:

In this example, we are going to find the elements of the array(i.e., the value of favGame.outdoorGames field) on the specified index using $arrayElemAt operator.

db.arrayExample.aggregate([
... {$match: {name: "Piku"}},
... {$project: {
... firstItem: {$arrayElemAt: ["$favGame.outdoorGames", 0]},
... item: {$arrayElemAt: ["$favGame.outdoorGames", 2]}}}])

Article Tags :

MongoDB

MongoDB-operators