reactivemongo.api.commands.AggregationFramework
$groups documents together to calculate aggregates on document collections. This command aggregates on multiple fields, and they must be named.
The fields to aggregate on, and the names they should be aggregated under.
the sequence of operators specifying aggregate calculation
$groups documents together to calculate aggregates on document collections. This command aggregates on multiple fields, and they must be named.
The fields to aggregate on, and the names they should be aggregated under.
the sequence of operators specifying aggregate calculation