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$firstN (aggregation accumulator)

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  • Syntax
  • Behavior
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  • Examples
$firstN

New in version 5.2.

Returns an aggregation of the first n elements within a group. The elements returned are meaningful only if in a specified sort order. If the group contains fewer than n elements, $firstN returns all elements in the group.

{
$firstN:
{
input: <expression>,
n: <expression>
}
}
  • input specifies the field(s) from the document to take the first n of. Input can be any expression.

  • n has to be a positive integral expression that is either a constant or depends on the _id value for $group. For details see group key example.

  • $firstN does not filter out null values.

  • $firstN converts missing values to null.

Consider the following aggregation that returns the first five documents from a group:

db.aggregate( [
{
$documents: [
{ playerId: "PlayerA", gameId: "G1", score: 1 },
{ playerId: "PlayerB", gameId: "G1", score: 2 },
{ playerId: "PlayerC", gameId: "G1", score: 3 },
{ playerId: "PlayerD", gameId: "G1"},
{ playerId: "PlayerE", gameId: "G1", score: null }
]
},
{
$group:
{
_id: "$gameId",
firstFiveScores:
{
$firstN:
{
input: "$score",
n: 5
}
}
}
}
] )

In this example:

  • $documents creates the literal documents that contain player scores.

  • $group groups the documents by gameId. This example has only one gameId, G1.

  • PlayerD has a missing score and PlayerE has a null score. These values are both considered as null.

  • The firstFiveScores field is specified using input : "$score" and returned as an array.

  • Since there is no sort criteria the first 5 score fields are returned.

[
{
_id: 'G1',
firstFiveScores: [ 1, 2, 3, null, null ]
}
]

Both $firstN and $topN accumulators can accomplish similar results.

In general:

  • If the documents coming into $group are already ordered, you should use $firstN.

  • If you're sorting and selecting the top n elements then you can use $topN to accomplish both tasks with one accumulator.

  • $firstN can be used as an aggregation expression, $topN cannot.

$firstN is supported as an aggregation expression.

For details on aggregation expression usage see Using $firstN as an Aggregation Expression.

$firstN is supported as a window operator.

Aggregation pipelines which call $firstN are subject to the 100 MB limit. If this limit is exceeded for an individual group, the aggregation fails with an error.

Consider a gamescores collection with the following documents:

db.gamescores.insertMany([
{ playerId: "PlayerA", gameId: "G1", score: 31 },
{ playerId: "PlayerB", gameId: "G1", score: 33 },
{ playerId: "PlayerC", gameId: "G1", score: 99 },
{ playerId: "PlayerD", gameId: "G1", score: 1 },
{ playerId: "PlayerA", gameId: "G2", score: 10 },
{ playerId: "PlayerB", gameId: "G2", score: 14 },
{ playerId: "PlayerC", gameId: "G2", score: 66 },
{ playerId: "PlayerD", gameId: "G2", score: 80 }
])

You can use the $firstN accumulator to find the first three scores in a single game.

db.gamescores.aggregate( [
{
$match : { gameId : "G1" }
},
{
$group:
{
_id: "$gameId",
firstThreeScores:
{
$firstN:
{
input: ["$playerId", "$score"],
n:3
}
}
}
}
] )

The example pipeline:

  • Uses $match to filter the results on a single gameId. In this case, G1.

  • Uses $group to group the results by gameId. In this case, G1.

  • Specifies the fields that are input for $firstN with input : ["$playerId"," $score"].

  • Uses $firstN to return the first three documents for the G1 game with n : 3.

The operation returns the following results:

[
{
_id: 'G1',
firstThreeScores: [ [ 'PlayerA', 31 ], [ 'PlayerB', 33 ], [ 'PlayerC', 99 ] ]
}
]

You can use the $firstN accumulator to find the first n input fields in each game.

db.gamescores.aggregate( [
{
$group:
{
_id: "$gameId", playerId:
{
$firstN:
{
input: [ "$playerId","$score" ],
n: 3
}
}
}
}
] )

The example pipeline:

  • Uses $group to group the results by gameId.

  • Uses $firstN to return the first three documents for each game with n: 3.

  • Specifies the fields that are input for $firstN with input : ["$playerId", "$score"].

The operation returns the following results:

[
{
_id: 'G1',
playerId: [ [ 'PlayerA', 31 ], [ 'PlayerB', 33 ], [ 'PlayerC', 99 ] ]
},
{
_id: 'G2',
playerId: [ [ 'PlayerA', 10 ], [ 'PlayerB', 14 ], [ 'PlayerC', 66 ] ]
}
]

Using a $sort stage earlier in the pipeline can influence the results of the $firstN accumulator.

In this example:

  • {$sort : { score : -1 } } sorts the highest scores to the back of each group.

  • firstN returns the three highest scores from front of each group.

db.gamescores.aggregate( [
{ $sort : { score : -1 } },
{
$group:
{ _id: "$gameId", playerId:
{
$firstN:
{
input: [ "$playerId","$score" ],
n: 3
}
}
}
}
] )

The operation returns the following results:

[
{
_id: 'G2',
playerId: [ [ 'PlayerD', 80 ], [ 'PlayerC', 66 ], [ 'PlayerB', 14 ] ]
},
{
_id: 'G1',
playerId: [ [ 'PlayerC', 99 ], [ 'PlayerB', 33 ], [ 'PlayerA', 31 ] ]
}
]

You can also assign the value of n dynamically. In this example, the $cond expression is used on the gameId field.

db.gamescores.aggregate([
{
$group:
{
_id: {"gameId": "$gameId"},
gamescores:
{
$firstN:
{
input: "$score",
n: { $cond: { if: {$eq: ["$gameId","G2"] }, then: 1, else: 3 } }
}
}
}
}
] )

The example pipeline:

  • Uses $group to group the results by gameId.

  • Specifies the fields that input for $firstN with input : "$score".

  • If the gameId is G2 then n is 1, otherwise n is 3.

The operation returns the following results:

[
{ _id: { gameId: 'G1' }, gamescores: [ 31, 33, 99 ] },
{ _id: { gameId: 'G2' }, gamescores: [ 10 ] }
]

You can also use $firstN as an aggregation expression.

In this example:

  • $documents creates the literal document that contains an array of values.

  • $project is used to return the output of $firstN.

  • _id is omited from the output with _id : 0.

  • $firstN uses the input array of [10, 20, 30, 40].

  • The first three elements of the array are returned for the input document.

db.aggregate( [
{
$documents: [
{ array: [10, 20, 30, 40] } ]
},
{ $project: {
firstThreeElements:{
$firstN:
{
input: "$array",
n: 3
}
}
}
}
] )

The operation returns the following results:

[
{ firstThreeElements: [ 10, 20, 30 ] }
]
←  $first (aggregation accumulator)$first (array operator) →
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