$arrayElemAt
Returns the element at the specified array index.
You can use $arrayElemAt for deployments hosted in the following environments:
MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud
MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB
$arrayElemAt has the following syntax:
{ $arrayElemAt: [ <array>, <idx> ] }The <array> expression can be any valid expression that resolves to an array.
The <idx> expression can be any valid expression that resolves to an integer.
For more information on expressions, see Expression Operators.
If the <idx> expression resolves to zero or a positive integer, $arrayElemAt returns the element at the idx position, counting from the start of the array.
If the <idx> expression resolves to a negative integer, $arrayElemAt returns the element at the idx position, counting from the end of the array.
If idx exceeds the array bounds, $arrayElemAt does not return a result.
If the <array> expression resolves to an undefined array, $arrayElemAt returns null.
Example
Results
{ $arrayElemAt: [ [ 1, 2, 3 ], 0 ] }
1
{ $arrayElemAt: [ [ 1, 2, 3 ], -2 ] }
2
{ $arrayElemAt: [ [ 1, 2, 3 ], 15 ] }
{ $arrayElemAt: [ "$undefinedField", 0 ] }
null
A collection named users contains the following documents:
db.users.insertMany( [ { _id: 1, name: "dave123", favorites: [ "chocolate", "cake", "butter", "apples" ] }, { _id: 2, name: "li", favorites: [ "apples", "pudding", "pie" ] }, { _id: 3, name: "ahn", favorites: [ "pears", "pecans", "chocolate", "cherries" ] }, { _id: 4, name: "ty", favorites: [ "ice cream" ] }] )The following example returns the first and last element in the favorites array:
db.users.aggregate([ { $project: { name: 1, first: { $arrayElemAt: [ "$favorites", 0 ] }, last: { $arrayElemAt: [ "$favorites", -1 ] } } }])The operation returns the following results:
[ { _id: 1, name: "dave123", first: "chocolate", "last" : "apples" }, { _id: 2, name: "li", first: "apples", "last" : "pie" }, { _id: 3, name: "ahn", first: "pears", "last" : "cherries" }, { _id: 4, name: "ty", first: "ice cream", "last" : "ice cream" }]RetroSearch is an open source project built by @garambo | Open a GitHub Issue
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