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$bottom (aggregation accumulator)

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  • Definition
  • Syntax
  • Behavior
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  • Examples
$bottom

New in version 5.2.

Returns the bottom element within a group according to the specified sort order.

{
$bottom:
{
sortBy: { <field1>: <sort order>, <field2>: <sort order> ... },
output: <expression>
}
}
Field
Necessity
Description
sortBy
Required
Specifies the order of results, with syntax similar to $sort.
output
Required
Represents the output for each element in the group and can be any expression.

Consider the following aggregation that returns the bottom document from a group of scores:

  • $bottom does not filter out null values.
  • $bottom converts missing values to null.
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",
playerId:
{
$bottom:
{
output: [ "$playerId", "$score" ],
sortBy: { "score": -1 }
}
}
}
}
] )

In this example:

  • $documents creates the literal documents that contain player scores.
  • $group groups the documents by gameId. This example has only one gameId, G1.
  • PlayerD has a missing score and PlayerE has a null score. These values are both considered as null.
  • The playerId and score fields are specified as output : ["$playerId"," $score"] and returned as array values.
  • Specify the sort order with sortBy: { "score": -1 }.
  • PlayerD and PlayerE tied for the bottom element. PlayerD is returned as the bottom score.
  • To have more deterministic tie breaking behavior for multiple null values, add more fields to``sortBy``.
[
{
_id: 'G1',
playerId: [ [ 'PlayerD', null ] ]
}
]

$bottom is not supported as a aggregation expression.

$bottom is supported as a window operator.

Aggregation pipelines which call $bottom 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 $bottom accumulator to find the bottom score in a single game.

db.gamescores.aggregate( [
{
$match : { gameId : "G1" }
},
{
$group:
{
_id: "$gameId",
playerId:
{
$bottom:
{
output: [ "$playerId", "$score" ],
sortBy: { "score": -1 }
}
}
}
}
] )

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 output for $bottom with output : ["$playerId"," $score"].
  • Uses sortBy: { "score": -1 } to sort the scores in descending order.
  • Uses $bottom to return the bottom score for the game.

The operation returns the following results:

[ { _id: 'G1', playerId: [ 'PlayerD', 1 ] } ]

You can use the $bottom accumulator to find the bottom score in each game.

db.gamescores.aggregate( [
{
$group:
{ _id: "$gameId", playerId:
{
$bottom:
{
output: [ "$playerId", "$score" ],
sortBy: { "score": -1 }
}
}
}
}
] )

The example pipeline:

  • Uses $group to group the results by gameId.
  • Uses $bottom to return the bottom score for each game.
  • Specifies the fields that are output for $bottom with output : ["$playerId", "$score"].
  • Uses sortBy: { "score": -1 } to sort the scores in descending order.

The operation returns the following results:

[
{ _id: 'G2', playerId: [ 'PlayerA', 10 ] },
{ _id: 'G1', playerId: [ 'PlayerD', 1 ] }
]
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