$minN (array operator)

Definition

$minN

Returns the n smallest values in an array.

Syntax

$minN has the following syntax:

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

Behavior

• You cannot specify a value of n less than 1.

• $minN 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, $minN 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 numeric elements are sorted before string 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 $minN operator to retrieve the two lowest scores for each player. The lowest scores are returned in the new field minScores created by $addFields.

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

The operation returns the following results:

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