Facetting with facet value containing a slash returns wrong facet counts

[mpauly]

Hi,
in one of our projects we are using django haystack alongside xapian-haystack for a paginated search with facetting. We are currently seeing the following problem:

If you facet for a property, where the property value contains a slash (e.g. hamburg/norden), the count of this facet always remains at 20, which is the number of items per page. The same does not happen for facetting values without a slash.

A typical URL that we use to facet would be ...?selected_facets=groups:hamburg/norden, together with a haystack FacetedSearchView.
If I manually check the value of self.queryset.filter(groups__in=['hamburg/norden']).count() in the view I get the correct count (>20).

The expected behavior would be, that the facetting count that I get from Haystack, i.e. the Xapian Backend also displays this number.

I took a look at the xapian-haystack source but was not able yet to track down what goes wrong, so any help is appreciated. We are using 2.1.1. Thanks in advance.