【Django - パーマリンク変更】unique + NOT NULLなカラムをあとから追加する方法

...
title = models.CharField(max_length=255)
slug = models.SlugField(max_length=30, unique=True)
content = MDTextField()
description = models.TextField(blank=True)
...

# python3 manage.py makemigrations blog

You are trying to add a non-nullable field 'slug' to product without a default; we can't do that (the database needs something to populate existing rows).
Please select a fix:
 1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
 2) Quit, and let me add a default in models.py

Select an option: 1
Please enter the default value now, as valid Python
The datetime and django.utils.timezone modules are available, so you can do e.g. timezone.now
Type 'exit' to exit this prompt
>>> 123
Migrations for 'blog':
  blog/migrations/0010_post_slug.py
    - Add field slug to post

# python3 manage.py migrate blog
Operations to perform:
  Apply all migrations: blog
Running migrations:
  Applying blog.0010_post_slug...Traceback (most recent call last):
  ...
  ...
  File "/usr/local/lib/python3.6/site-packages/django/db/backends/sqlite3/base.py", line 328, in execute
    return Database.Cursor.execute(self, query, params)
sqlite3.IntegrityError: UNIQUE constraint failed: blog_post.post_slug

# python3 manage.py makemigrations

No changes detected

...
title = models.CharField(max_length=255)
slug = models.SlugField(max_length=30, null=True, default=123) ←　ここを変更
content = MDTextField()
description = models.TextField(blank=True)
...

python3 manage.py makemigrations blog

Migrations for 'blog':
  blog/migrations/0010_post_slug.py
    - Add field slug to post

# Generated by Django 3.0.2 on 2020-xx-xx xx:xx

from django.db import migrations, models

+def set_default_slug(apps, schema_editor):　　←　これを追加
+    post_model = apps.get_model('blog', 'Post')
+    for row in post_model.objects.all():
+        row.slug = '{}'.format(row.title)
+        row.save()

class Migration(migrations.Migration):

    dependencies = [
        ('blog', '0009_category_parent'),
    ]

    operations = [
        migrations.AddField(
            model_name='post',
            name='slug',
            field=models.SlugField(default='123', max_length=30, null=True),
            preserve_default=False,
        ),
+        migrations.RunPython(set_default_slug, reverse_code=migrations.RunPython.noop),　←　これを追加
    ]

# Generated by Django 3.0.2 on 2020-xx-xx xx:xx

from django.db import migrations, models

+def set_default_slug(apps, schema_editor):
    post_model = apps.get_model('blog', 'Post')
    for row in post_model.objects.all():
        row.slug = '{}'.format(row.title)
        row.save()

class Migration(migrations.Migration):

    dependencies = [
        ('blog', '0009_category_parent'),
    ]

   operations = [
        migrations.AddField(
            model_name='post',
            name='slug2',
            field=models.SlugField(default='123', max_length=30, null=True),
            preserve_default=False,
        ),
        migrations.RunPython(set_default_slug, reverse_code=migrations.RunPython.noop),

+        migrations.AlterField(　　←　slugカラムを修正
+            model_name='post',
+            name='slug2',
+            field=models.CharField(max_length=30, unique=True, null=False, blank=False),  ←　NOT NULLなユニーク制約を追加
+            preserve_default=False,
+        ),
    ]

# python3 manage.py migrate blog

Operations to perform:
  Apply all migrations: blog
Running migrations:
  Applying blog.0010_post_slug... OK