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: w3wiki
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]}}}])
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