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$maxN (acumulator operator)

Definition

$maxN: New in version 5.2.
Returns the n largest values in an array.

Tip

$minN

Syntax

$maxN has the following syntax:

{ $maxN: { n: <expression>, input: <expression> } }

Field	Description
`n`	An expression that resolves to a positive integer. The integer specifies the number of array elements that `$maxN` returns.
`input`	An expression that resolves to the array from which to return the maximal `n` elements.

Behavior

You cannot specify a value of n less than 1.
$maxN filters out null values found in the input array.
If the specified n is greater than or equal to the number of elements in the input array, $maxN returns all elements in the input array.
If input resolves to a non-array value, the aggregation operation errors.
If input contains both numeric and string elements, the string elements are sorted before numeric elements according to the BSON comparison order.

Example

Create a scores collection with the following documents:

db.scores.insertMany([
    { "playerId" : 1, "score" : [ 1, 2, 3 ] },
    { "playerId" : 2, "score" : [ 12, 90, 7, 89, 8 ] },
    { "playerId" : 3, "score" : [ null ] },
    { "playerId" : 4, "score" : [ ] }
    { "playerId" : 5, "score" : [ 1293, "2", 3489, 9 ]}
])

The following example uses the $maxN operator to retrieve the two highest scores for each player. The highest scores are returned in the new field maxScores created by $addFields.

db.scores.aggregate([
   { $addFields: { maxScores: { $maxN: { n: 2, input: "$score" } } } }
])

The operation returns the following results:

[{
  "playerId": 1,
  "score": [ 1, 2, 3 ],
  "maxScores": [ 3, 2 ]
},
{
  "playerId": 2,
  "score": [ 12, 90, 7, 89, 8 ],
  "maxScores": [ 90, 89 ]
},
{
  "playerId": 3,
  "score": [ null ],
  "maxScores": [ ]
},
{
  "playerId": 4,
  "score": [ ],
  "maxScores": [ ]
},
{
  "playerId": 5,
  "score": [ 1293, "2", 3489, 9 ],
  "maxScores": [ "2", 3489 ]
}]

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