How to include facets that have a count of 0 in facet_counts() with django-haystack when narrowing or filtering?

Question

I am using django-haystack with elasticsearch. I want to show sport sessions on a website. For now I have two facets; sport and training level. Both of them are a list of check boxes and are 'OR' oriented, which means that multiple can be selected.

For the example, there are 3 training options with 4 different training each. All the basketball training is for beginners and all football and dancing training is for advanced. This would result in the following:

Training options: Basketball (4), Football (4), Dancing (4)

Training levels: Beginner (4), Advanced (8)

If I click on basketball, the following result is displayed:

Basketball (3), Football (3), Dancing (3)

Beginner (3)

In this case the advanced is left out. What I want though is:

Basketball (3), Football (3), Dancing (3)

Beginner (3), Advanced (0)

I use the following code:

from haystack.forms import FacetedSearchForm

class FacetedSearchSportForm(FacetedSearchForm):
    def __init__(self, *args, **kwargs):
        data = dict(kwargs.get("data", []))

        self.sports = data.get('sports', [])
        self.level = data.get('level', [])

        super().__init__(*args, **kwargs)

    def search(self):
        sqs = super().search()

        if not self.is_valid():
            return self.no_query_found()

        if self.sports:
            query = None

            for sport in self.sports:
                if query:
                    query += u' OR '
                else:
                    query = u''
                    query += u'"%s"' % sqs.query.clean(sport)

            sqs = sqs.narrow(u'sports_exact:%s' % query)

        if self.level:
            query = None

            for level in self.level:
                if query:
                    query += u' OR '
                else:
                    query = u''
                    query += u'"%s"' % sqs.query.clean(level)

            sqs = sqs.narrow(u'level_exact:%s' % query)

        return sqs

If I replace sqs.narrow with a sqs.filter the same happens. Does anyone know how I can get to include the count of 0 when filtering with facets?

Thanks!