Embedding aggregation expressions

The $expr operator allows embedding of aggregation expressions in a normal query (or in a $match stage). Which of the following expressions counts the number of laureate documents with string-valued bornCountries when passed to db.laureates.count_documents?

You can assume (and check!) that the following is true:

assert all(isinstance(v, str) for v in db.laureates.distinct("bornCountry"))

Possible Answers

{"bornCountry": {"$in": db.laureates.distinct("bornCountry")}}
{"$expr": {"$in": ["$bornCountry", db.laureates.distinct("bornCountry")]}}
{"$expr": {"$eq": [{"$type": "$bornCountry"}, "string"]}}
{"bornCountry": {"$type": "string"}}
All of the above

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Exercise

Embedding aggregation expressions

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