class Non_ONCO(CommonField):

     def clean(self):
          if not self.id:
               slug = slugify(self.libelle)
               if Non_ONCO.objects.filter(slug_libelle=slug).exists():
                    raise ValidationError(u"Ce libellé existe déjà")

     def save(self,force_insert=False,force_update=False):
          self.slug_libelle = slugify(self.libelle)
          super(Non_ONCO,self).save(force_insert=False,force_update=False)