django-filters .. paring down the choices in a related widget.

filtersets.py

from django_filters import FilterSet

class MyModelFilterSet(FilterSet):
    class Meta:
        model = MyModel
        fields = ['my_relation']  # internally this is a reverse relation, so MyModelInstance.my_relation_set.all()
        
# How do I get it so that the ModelChoiceFilter (actually a ModelMultipleChoiceFilter!) is only 
# those items which exist given the FilterSet's original queryset (self.queryset.all())

Ended up doing it in the filterset's init method. Bit nasty, but it works.