How does mongodb count the number of subdocuments?

  mongodb, question

As the topic is.
MongoDB can count the number of documents in a collection using the Count method, which belongs to the collection method, but how to count the number of embedded subdocuments? I have checked the official documents and found no suitable method in the aggregation framework. I hope someone can give me some advice.

{$project: {‘cnt’: {$size: ‘$Scores’}}}

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