How to write tests in django?

Question

Django

Just started writing tests. The project uses django-mptt. How to write tests for it? SQLite.
In this test, user2 becomes a child of user1 and user1.invite_period_expire_date is set to user2.contract_date+timedelta(days=30). Only the user2.get_ancestors() call returns an empty array. The attributes only list the fields that django complained about. Now the tests just don't pass.

from django.test import TestCase
from models import TreeNode
from datetime import date, timedelta


class TreeTest(TestCase):
    def test_invite_period_works_correctly(self):
        user1 = TreeNode.objects.create(id=4,
                         date_of_birth=date.today(),
                         passport_address_id=1,
                         identity_expire_date=date.today(),
                         contract_date=date(2014, 1, 1),
                         itn=1, account_id=1, level=1, lft=1, rght=4, tree_id=1).save()
        user2 = TreeNode.objects.create(id=15,
                         date_of_birth=date.today(),
                         passport_address_id=1,
                         identity_expire_date=date.today(),
                         contract_date=date(2014, 1, 2),
                         itn=2, account_id=2, level=2, lft=1, rght=3, tree_id=1)\
            .insert_at(TreeNode.objects.get(id=4),, save=True)
        self.assertEqual(user2.get_parent_of_level(1).invite_period_expire_date, user2.contract_date + timedelta(days=30))

tested model

class TreeNode(MPTTModel):

    id = models.CharField(verbose_name='Клиентский ID', max_length=10, primary_key=True)
    parent = TreeForeignKey('self', verbose_name='ID пригласившего', null=True, blank=True, related_name='child')

    contract_date = models.DateField(null=False, blank=False)
    invite_period_expire_date = models.DateField(null=True, blank=True)

    def save(self, *args, **kwargs):
        super(TreeNode, self).save(*args, **kwargs)
        if self.get_parent_of_level(1):
            if not self.get_parent_of_level(1).invite_period_expire_date:
                self.get_parent_of_level(1).start_invite_period(self.contract_date)
    
    def start_invite_period(self, start_date):
        self.invite_period_expire_date = start_date + invite_period
        self.save()

    def get_parent_of_level(self, level):
        if self.get_level() - level >= 0:
            return self.get_ancestors().get(level=self.get_level() - level)
        return None