This member is added by an implicit conversion from PipelineOperator to
any2stringadd[PipelineOperator] performed by method any2stringadd in scala.Predef.
This member is added by an implicit conversion from PipelineOperator to
StringFormat[PipelineOperator] performed by method StringFormat in scala.Predef.
(Since version 0.12-RC5) Use reactivemongo.api.collections.GenericCollection.aggregateWith