Django: extend get_object for class-based views

Being a non-expert Python programmer, I'm looking for feedback on the way I extended the get_object method of Django's SingleObjectMixin class.

For most of my Detail views, the lookup with a pk or slugfield is fine - but in some cases, I need to retrieve the object based on other (unique) fields, e.g. "username". I subclassed Django's DetailView and modified the get_object method as follows:

# extend the method of getting single objects, depending on model
def get_object(self, queryset=None):

    if self.model != mySpecialModel:
        # Call the superclass and do business as usual 
        obj = super(ObjectDetail, self).get_object()
        return obj

    else:
        # add specific field lookups for single objects, i.e. mySpecialModel
        if queryset is None:
            queryset = self.get_queryset()

        username = self.kwargs.get('username', None)
        if username is not None:
            queryset = queryset.filter(user__username=username)
        # If no username defined, it's an error.
        else:
            raise AttributeError(u"This generic detail view %s must be called with "
                                 u"an username for the researcher."
                                 % self.__class__.__name__)

        try:
            obj = queryset.get()
        except ObjectDoesNotExist:
            raise Http404(_(u"No %(verbose_name)s found matching the query") %
                          {'verbose_name': queryset.model._meta.verbose_name})
        return obj

Is this good practise? I try to have one Subclass of Detailview, which adjusts to differing needs when different objects are to be retrieved - but which also maintains the default behavior for the common cases. Or is it better to have more subclasses for the special cases?

Thanks for your advice!