Returns an aggregation of the top n elements within a group, according to the specified sort order. If the group contains fewer than n elements, $topN returns all elements in the group.
The input array
The right-hand side expression value of the operator
The options to use for this operation
Returns an aggregation of the top n elements within a group, according to the specified sort order. If the group contains fewer than n elements, $topN returns all elements in the group.