Add aggregations representing the facets selected, including potential
        filters.

def aggregate(self, search):
        """
        Add aggregations representing the facets selected, including potential
        filters.
        """
        for f, facet in iteritems(self.facets):
            agg = facet.get_aggregation()
            agg_filter = MatchAll()
            for field, filter in iteritems(self._filters):
                if f == field:
                    continue
                agg_filter &= filter
            search.aggs.bucket(
                '_filter_' + f,
                'filter',
                filter=agg_filter
            ).bucket(f, agg)