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$score (aggregation)

New in version 8.2.

Definition

$score: $score computes and returns a new score as metadata. It also optionally normalizes the input scores, by default to a range between zero and one.

Syntax

The stage has the following syntax:

{
  $score: {
    score: <expression>,
    scoreDetails: <boolean>,
    normalization: "none|sigmoid|minMaxScaler",
    weight: <expression>
  }
}

Fields

$score takes the following fields:

Field

Type

Description

score

Expression

Computes a new value from the input scores and stores the value in the $meta keyword score. Returns an error for non-numeric inputs.

scoreDetails

Boolean

Default is false. Specifies if $score computes and populates the $scoreDetails metadata field for each output document. See scoreDetails for more information on this field.

normalization

String

Optional. Normalizes the score to the range of 0 to 1. Value can be:

none - Doesn't normalize.
sigmoid - Applies the following $sigmoid expression:
```
1 / (1 + e^-x)
```
Here, x is the score to normalize.
If omitted, defaults to none.
minMaxScaler - to apply the $minMaxScaler window function:
```
 (s - min_s) / (max_s - min_s)
```
Where:
- s is the score.
- min_s is the minimum score.
- max_s is the maximum score.

weight

Double

Optional. Number to multiply the score expression by after normalization.

Behavior

The output documents from a $score are the same as the input documents, but includes additional computed score as metadata.

If you specify multiple $score stages in the pipeline, the last $score stage in the pipeline overrides the score metadata from previous $score stages.

scoreDetails

If you set scoreDetails to true, $score creates a scoreDetails metadata field for each document. The scoreDetails field contains information about the final ranking.

Note

When you set scoreDetails to true, $score sets the scoreDetails metadata field for each document but does not automatically output the scoreDetails metafield.

To view the scoreDetails metadata field, you must either:

use a $project stage after $score to project the scoreDetails field
use a $addFields stage after $score to add the scoreDetails field to your pipeline output

The scoreDetails field contains the following subfields:

Field	Description
`value`	The calculated score.
`description`	A description in string format that explains how the `$score` operationMetrics calculates the final score.
`rawScore`	The unweighted raw (not normalized) score.
`normalization`	One of the following normalization values: "none", "sigmoid", "minMaxScaler".
`weight`	The weight of the input pipeline, which must be in range [0,1].
`expression`	The value of `$score.score` in string format.
`details`	An empty array, as `$score` cannot take any input pipelines.

Warning

MongoDB does not guarantee any specific output format for scoreDetails.

For example, the following code blocks shows the scoreDetails field for a $score operation with an $add expression:

 {
  _id: ObjectId('55f5a192d4bede9ac365b257'),
  scoreDetails: {
    value: 0.5,
    description: 'the score calculated from multiplying a weight in the range [0,1] with either a normalized or nonnormalized value:',
    rawScore: 145,
    normalization: 'sigmoid',
    weight: 0.5,
    expression: '{ string: { $add: [ 'field', 'myField' ] } }',
    details: []
  }
},

Back

$sample

$scoreFusion