Returns an aggregation of the minimum value n elements within a group. If the group contains fewer than n elements, $minN returns all elements in the group.
5.2
https://www.mongodb.com/docs/manual/reference/operator/aggregation/minN/#mongodb-group-grp.-minN
Returns an aggregation of the minimum value n elements within a group. If the group contains fewer than n elements, $minN returns all elements in the group.