Использую Django MPTT для создания древовидных комментариев. И у меня возник вопрос как можно ограничить максимальный уровень вложенности комментариев например до 5.

from django.conf import settings from django.db import models from mptt.models import MPTTModel, TreeForeignKey from django.contrib.contenttypes.fields import GenericForeignKey from django.contrib.contenttypes.models import ContentType from django.utils.translation import ugettext_lazy as _ from django.utils import timezone  COMMENT_MAX_LENGTH = getattr(settings, 'COMMENT_MAX_LENGTH', 3000)   class CommentManager(models.Manager):      def filter_by_instance(self, instance):         content_type = ContentType.objects.get_for_model(instance.__class__)         object_id = instance.id         qs = super(CommentManager, self).filter(             content_type=content_type, object_id=object_id)         return qs   class CommentAbstractModel(MPTTModel):     content_type = models.ForeignKey(         ContentType,         verbose_name=_('content type'),         related_name="content_type_set_for_%(class)s",         on_delete=models.CASCADE)     object_id = models.PositiveIntegerField()     content_object = GenericForeignKey('content_type', 'object_id')      class Meta:         abstract = True   class Comment(CommentAbstractModel):     user = models.ForeignKey(         settings.AUTH_USER_MODEL,         verbose_name=_('user'),         related_name="%(class)s_comments",         on_delete=models.CASCADE)     comment = models.TextField(         _('comment'),         max_length=COMMENT_MAX_LENGTH)     karma = models.SmallIntegerField(         default=0)     parent = TreeForeignKey(         'self',         null=True,         blank=True,         related_name=_('children'),         db_index=True)     is_public = models.BooleanField(         _('is public'),         default=True,         help_text=_('Uncheck this box to make the comment effectively '                     'disappear from the site.'))     is_removed = models.BooleanField(         _('is removed'),         default=False,         help_text=_('Check this box if the comment is inappropriate. '                     'A "This comment has been removed" message will '                     'be displayed instead.'))     created_at = models.DateTimeField(         default=timezone.now,         verbose_name=_(u'Creation date'))     updated_at = models.DateTimeField(         blank=True,         null=True,         verbose_name=_(u'Modification date'))      def update(self):         self.updated_at = timezone.now()         self.save()      objects = CommentManager()      class Meta(CommentAbstractModel.Meta):         ordering = ['-created_at']         verbose_name = _('comment')         verbose_name_plural = _('comments')      def __str__(self):         return "%s: %s..." % (self.user.username, self.comment[:50])