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$lastN (aggregation accumulator)![](/docs/v5.3/assets/link.svg)
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Definition![](/docs/v5.3/assets/link.svg)
Syntax![](/docs/v5.3/assets/link.svg)
{ $lastN: { input: <expression>, n: <expression> } }
input
specifies the field(s) from the document to take the lastn
of. Input can be any expression.n
has to be a positive integral expression that is either a constant or depends on the_id
value for$group
. For details see group key example.
Behavior![](/docs/v5.3/assets/link.svg)
Null and Missing Values![](/docs/v5.3/assets/link.svg)
$lastN
does not filter out null values.$lastN
converts missing values to null.
Consider the following aggregation that returns the last 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", lastFiveScores: { $lastN: { input: "$score", n: 5 } } } } ] )
In this example:
$documents
creates the literal documents that contain player scores.$group
groups the documents bygameId
. This example has only onegameId
,G1
.PlayerD
has a missing score andPlayerE
has a nullscore
. These values are both considered as null.The
lastFiveScores
field is specified usinginput : "$score"
and returned as an array.Since there is no sort criteria the last 5
score
fields are returned.
[ { _id: "G1", lastFiveScores: [ 1, 2, 3, null, null ] } ]
Comparison of $lastN
and $bottomN
![](/docs/v5.3/assets/link.svg)
Both $lastN
and $bottomN
accumulators can accomplish similar
results.
In general:
If the documents coming into
$group
are already ordered, you should use$lastN
.If you're sorting and selecting the bottom
n
elements then you can use$bottomN
to accomplish both tasks with one accumulator.$lastN
can be used as an aggregation expression,$bottomN
cannot.
Restrictions![](/docs/v5.3/assets/link.svg)
Window Function and Aggregation Expression Support![](/docs/v5.3/assets/link.svg)
$lastN
is supported as an
aggregation expression.
For details on aggregation expression usage see Using $lastN as an Aggregation Expression.
$lastN
is supported as a
window operator
.
Memory Limit Considerations![](/docs/v5.3/assets/link.svg)
Aggregation pipelines which call $lastN
are subject to the
100 MB limit. If this
limit is exceeded for an individual group, the aggregation fails
with an error.
Examples![](/docs/v5.3/assets/link.svg)
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 } ])
Find the Last Three Player Scores for a Single Game![](/docs/v5.3/assets/link.svg)
You can use the $lastN
accumulator to find the last three scores
in a single game.
db.gamescores.aggregate( [ { $match : { gameId : "G1" } }, { $group: { _id: "$gameId", lastThreeScores: { $lastN: { input: ["$playerId", "$score"], n:3 } } } } ] )
The example pipeline:
Uses
$match
to filter the results on a singlegameId
. In this case,G1
.Uses
$group
to group the results bygameId
. In this case,G1
.Specifies the fields that are output from
$lastN
withoutput : ["$playerId"," $score"]
.Uses
$lastN
to return the last three documents for theG1
game withn : 3
.
The operation returns the following results:
[ { _id: "G1", lastThreeScores: [ [ "PlayerB", 33 ], [ "PlayerC", 99 ], [ "PlayerD", 1 ] ] } ]
Finding the Last Three Player Scores Across Multiple Games![](/docs/v5.3/assets/link.svg)
You can use the $lastN
accumulator to find the last n
input fields in each game.
db.gamescores.aggregate( [ { $group: { _id: "$gameId", playerId: { $lastN: { input: [ "$playerId","$score" ], n: 3 } } } } ] )
The example pipeline:
Uses
$group
to group the results bygameId
.Uses
$lastN
to return the last three documents for each game withn: 3
.Specifies the fields that are input for
$lastN
withinput : ["$playerId", "$score"]
.
The operation returns the following results:
[ { _id: 'G2', playerId: [ [ 'PlayerB', 14 ], [ 'PlayerC', 66 ], [ 'PlayerD', 80 ] ] }, { _id: 'G1', playerId: [ [ 'PlayerB', 33 ], [ 'PlayerC', 99 ], [ 'PlayerD', 1 ] ] } ]
Using $sort
With $lastN
![](/docs/v5.3/assets/link.svg)
Using a $sort
stage earlier in the pipeline can influence the
results of the $lastN
accumulator.
In this example:
{$sort : { score : -1 } }
sorts the highest scores to the back of each group.lastN
returns the three lowest scores from the back of each group.
db.gamescores.aggregate( [ { $sort : { score : -1 } }, { $group: { _id: "$gameId", playerId: { $lastN: { input: [ "$playerId","$score" ], n: 3 } } } } ] )
The operation returns the following results:
[ { _id: 'G2', playerId: [ [ 'PlayerC', 66 ], [ 'PlayerB', 14 ], [ 'PlayerA', 10 ] ] }, { _id: 'G1', playerId: [ [ 'PlayerB', 33 ], [ 'PlayerA', 31 ], [ 'PlayerD', 1 ] ] } ]
Computing n
Based on the Group Key for $group
![](/docs/v5.3/assets/link.svg)
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: { $lastN: { input: "$score", n: { $cond: { if: {$eq: ["$gameId","G2"] }, then: 1, else: 3 } } } } } } ] )
The example pipeline:
Uses
$group
to group the results bygameId
.Specifies the fields that input for
$lastN
withinput : "$score"
.If the
gameId
isG2
thenn
is 1, otherwisen
is 3.
The operation returns the following results:
[ { _id: { gameId: "G2" }, gamescores: [ 10 ] }, { _id: { gameId: "G1" }, gamescores: [ 33, 31, 1 ] } ]
Using $lastN
as an Aggregation Expression![](/docs/v5.3/assets/link.svg)
You can also use $lastN
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$lastN
._id
is omited from the output with_id : 0
.$lastN
uses the input array of[10, 20, 30, 40]
.The last three elements of the array are returned for the input document.
db.aggregate( [ { $documents: [ { array: [10, 20, 30, 40] } ] }, { $project: { lastThreeElements:{ $lastN: { input: "$array", n: 3 } } } } ] )
The operation returns the following results:
[ { lastThreeElements: [ 20, 30, 40 ] } ]