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MongoDB Aggregation Framework way for "select count (distinct fieldName) from someTable" for large collections
//equivalent of "select count (distinct fieldName) from someTable"
db.someCollection.aggregate([{ $group: { _id: "$fieldName"} },{ $group: { _id: 1, count: { $sum: 1 } } } ])

Found this ^ when i wanted this: https://gist.github.com/clarkenheim/fa0f9e5400412b6a0f9d
returns a result with the distinct field names, and a count of the documents in the collection which have each distinct field value.

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