$arrayElemAt (aggregation)¶

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Definition
Behavior
Example

Definition¶

$arrayElemAt¶

3.2 新版功能.

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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