def parse_secondary_types(self, request):
        """
        Return secondary event types that occurrences must belong to, or `None`
        if there is no constraint on secondary type.
        """
        if request.GET.getlist('secondary_types'):
            return EventType.objects.filter(
                slug__in=request.GET.getlist('secondary_types'))
        return None
Return secondary event types that occurrences must belong to, or `None`
        if there is no constraint on secondary type.