Score the Documents in the Results

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 stage in your aggregation pipeline.

After the $search stage, in the $project stage, the score field takes the $meta expression, which requires the searchScore value. You can also specify the searchScoreDetails value for the scoreDetails field $meta expression for a detailed breakdown of the score.

After the $vectorSearch stage, in the $project stage, the score field takes the $meta expression, which requires the vectorSearchScore value to return the score of each document in your vector search results.

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db.movies.aggregate([
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  {
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    "$search": {
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      "text": {
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        <operator-specification>
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      }
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    }
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  },
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  {
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    "$project": {
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      "<field-to-include>": 1,
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      "<field-to-exclude>": 0,
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      "score": { "$meta": "searchScore" }
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    }
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  }
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])

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db.movies.aggregate([
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  {
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    "$search": {
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      "text": {
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        <operator-specification>
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      },
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      "scoreDetails": true
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    }
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  },
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  {
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    "$project": {
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      "<field-to-include>": 1,
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      "<field-to-exclude>": 0,
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      "scoreDetails": { "$meta": "searchScoreDetails" }
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    }
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  }
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])

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db.movies.aggregate([
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  {
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    "$vectorSearch": {
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      <query-syntax>
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    }
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  },
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  {
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    "$project": {
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      "<field-to-include>": 1,
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      "<field-to-exclude>": 0,
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      "score": { "$meta": "vectorSearchScore" }
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    }
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  }
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])

After a $project stage, you don't need to include a descending $sort because Atlas Search returns the documents from highest score to lowest. However, if multiple documents in the results have identical scores, the ordering of the documents in the results is non-deterministic. If you want the results to have a determined order, we recommend that you include the sort option in your $search stage to sort the results by a unique field. If you don't specify a unique field, Atlas Search defaults to sorting the results arbitrarily when the results have an identical score. You can use the sort option to also return an ascending sort of the results by score. To learn more, see Sort Atlas Search Results and Sort by Score Examples.

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