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- $arrayElemAt (aggregation)
$arrayElemAt (aggregation)¶
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Definition¶
- $arrayElemAt¶
New in version 3.2.
Returns the element at the specified array index.
$arrayElemAt has the following syntax:
{ $arrayElemAt: [ <array>, <idx> ] }
The <array> expression can be any valid expression as long as it resolves to an array.
The <idx> expression can be any valid expression as long as it resolves to an integer.
- If positive, $arrayElemAt returns the element at the idx position, counting from the start of the array.
- If negative, $arrayElemAt returns the element at the idx position, counting from the end of the array.
If the idx exceeds the array bounds, $arrayElemAt does not return any result.
For more information on expressions, see Expressions.
Behavior¶
For more information on expressions, see Expressions.
Example | Results |
---|---|
{ $arrayElemAt: [ [ 1, 2, 3 ], 0 ] } | 1 |
{ $arrayElemAt: [ [ 1, 2, 3 ], -2 ] } | 2 |
{ $arrayElemAt: [ [ 1, 2, 3 ], 15 ] } |
Example¶
A collection named users contains the following documents:
{ "_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" }
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