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Every document returned by an Atlas Search query is assigned a score based on relevance, and the documents included in a result set are returned in order from highest score to lowest.

Many factors can influence a document's score, including:

  • The position of the search term in the document,
  • The frequency of occurrence of the search term in the document,
  • The type of operator the query uses,
  • The type of analyzer the query uses.

The score assigned to a returned document is part of the document's metadata. You can include each returned document's score along with the result set by using a $project in your aggregation pipeline.

The following example query uses a $project stage to add a field named score to the returned documents:

1db.movies.aggregate([
2 {
3 $search: {
4 "text": {
5 "query": "Helsinki",
6 "path": "plot"
7 }
8 }
9 },
10 {
11 $project: {
12 plot: 1,
13 title: 1,
14 score: { $meta: "searchScore" }
15 }
16 }
17])

More information about the Lucene scoring algorithm can be found in the Lucene documentation.

The following score modifying options are available to all operators. For details and examples, click any of the following options:

The boost option multiplies a result's base score by a given number or the value of a numeric field in the documents.

Note
  • The boost and constant options can't be used together.
  • The boost option with path is the same as multiplying using the function option with a path expression.

The boost option takes the following fields:

Field
Type
Necessity
Description
value
float
Conditional
Number to multiply the default base score by. Value must be a positive number. Either value or path is required, but you can't specify both.
path
string
Conditional
Name of the numeric field whose value to multiply the default base score by. Either path or value is required, but you can't specify both.
undefined
float
Optional
Numeric value to substitute for path if the numeric field specified through path is not found in the documents. If omitted, defaults to 0. You can specify this only if you specify path.

The following examples use the movies collection in the sample_mflix database. If you have the sample dataset on your cluster, you can create the Atlas Search default index and run the example queries on your cluster.

The sample queries include a $project stage to exclude all fields except title and score.

The constant option replaces the base score with a specified number.

Note

The constant and boost options may not be used together.

The following example uses the default index on the sample_mflix.movies collection to query the title field. In the query, the text operator uses``score`` with the constant option to replace all score results with 5.

1db.movies.aggregate([
2 {
3 $search: {
4 "text": {
5 "query": "tower",
6 "path": "title",
7 "score": { "constant": { "value": 5 } }
8 }
9 }
10 },
11 {
12 $project: {
13 "_id": 0,
14 "title": 1,
15 "score": { "$meta": "searchScore" }
16 }
17 }
18])

The above query returns the following results, in which the score is replaced with the specified constant value:

1{ "title": "Tower Heist", "score": 5 }
2{ "title": "Tokyo Tower", "score": 5 }
3{ "title": "The Leaning Tower", "score": 5 }
4{ "title" : "Tower Block", "score" : 5 }
5{ "title": "Ivory Tower", "score": 5 }

The function option allows you to alter the final score of the document using a numeric field . You can specify the numeric field for computing the final score through an expression.

Use the following expressions with the function option to alter the final score of the document:

  • Arithmetic expressions, which add or multiply a series of numbers.
  • Constant expressions, which allow a constant number in the function score.
  • Path expressions, which incorporate an indexed numeric field value into a function score.
  • Score expressions, which return the relevance score assigned by Atlas Search.
  • Unary expressions, which are expressions that take a single argument. In Atlas Search, you can calculate the log10(x) or log10(x+1) of a specified number.

The following examples use the default index on the sample_mflix.movies collection to query the title field. The sample queries include a $limit stage to limit the output to 5 results and a $project stage to exclude all fields except title and score.

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