Returns an aggregation of the first n elements within a group. The elements returned are meaningful only if in a specified sort order.
If the group contains fewer than n elements, $firstN returns all elements in the group.
Param: collection
The input array
Param: expr
The right-hand side expression value of the operator
Returns an aggregation of the first n elements within a group. The elements returned are meaningful only if in a specified sort order. If the group contains fewer than n elements, $firstN returns all elements in the group.