models.py

class Lead(models.Model):
   # basic lead gen stuff goes here


class Step(Default):
    lead = models.ForeignKey(Lead)
    active = models.BooleanField(default=False)
    comment = models.TextField(blank=True)

    def save(self, force_insert=False, force_update=False, using=None, update_fields=None):
        getattr(self.lead, self.__class__.__name__.lower() + '_set').update(active=False)
        self.active = True
        super().save(force_insert, force_update, using, update_fields)

    def __str__(self):
        return self.name

    class Meta:
        abstract = True


class InternalStage(Step):
    AVAILABLE_INTERNAL_STAGES = (
        ('pending', 'Pending'),
        ('internally_approved', 'Internally Approved'),
        ('open', 'Open'),
    )

    name = models.CharField(max_length=64, choices=AVAILABLE_INTERNAL_STAGES)

class LeadQuerySet(models.query.QuerySet):

    def without_stage_of(self, stage, *names):
        """
        Exclude stage(s).

        :param stage: str
        :param names: str
        :return: QuerySet
        """
        stage = stage.replace('_', '')
        f = Q(**{stage + '__active': True})
        for name in names:
            f = f & ~Q(**{stage + '__name': name})
        return self.filter(f)