method inserts aggregated objects into MongoDB
            :return number_of_aggregated_objects

def _flush_aggregated_objects(self):
        """ method inserts aggregated objects into MongoDB
            :return number_of_aggregated_objects """
        if len(self.aggregated_objects) == 0:
            # nothing to do
            return 0

        number_of_aggregated_objects = len(self.aggregated_objects)
        self.logger.info('Aggregated {0} documents. Performing flush.'.format(number_of_aggregated_objects))

        for key in self.aggregated_objects:
            document = self.aggregated_objects[key]
            mongo_pk = self._mongo_sink_key(*key)
            self.ds.update(self.sink, mongo_pk, document)

        self.logger.info('Flush successful.')

        del self.aggregated_objects
        self.aggregated_objects = dict()
        gc.collect()
        return number_of_aggregated_objects