How can I select a number of records per a specific field using mongodb?

Question

I have a collection of documents in mongodb, each of which have a \"group\" field that refers to a group that owns the document. The documents look like this:

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Answer 1

you need aggregation framework $group stage piped in a $limit stage... you want also to $sort the records in some ways or else the limit will have undefined behaviour, the returned documents will be pseudo-random (the order used internally by mongo)

something like that: db.collection.aggregate([{$group:...},{$sort:...},{$limit:...}])

here there is the documentation if you want to know more