Передача случайных уникальных значений по умолчанию при миграции на юг

В django 1.7+ вы можете сделать следующее. Сначала он добавляет поле без индексации и без уникальности. Затем он присваивает уникальные значения (я основал их на имени и использовал метод slugify, который вам нужно создать) и, наконец, снова изменяет поле, чтобы добавить индекс и уникальные атрибуты.

from django.db import migrations
import re
import django.contrib.postgres.fields
from common.utils import slugify
import django.core.validators


def set_slugs(apps, schema_editor):
    categories = apps.get_model("myapp", "Category").objects.all()
    for category in categories:
        category.slug = slugify(category.name)
        category.save()


class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0034_auto_20150906_1936'),
    ]

    operations = [
        migrations.AddField(
            model_name='category',
            name='slug',
            field=models.CharField(max_length=30, validators=[django.core.validators.MinLengthValidator(2), django.core.validators.RegexValidator(re.compile('^[0-9a-z-]+$'), 'Enter a valid slug.', 'invalid')], help_text='Required. 2 to 30 characters and can only contain a-z, 0-9, and the dash (-)', unique=False, db_index=False, null=True),
            preserve_default=False,
        ),
        migrations.RunPython(set_slugs),
        migrations.AlterField(
            model_name='category',
            name='slug',
            field=models.CharField(help_text='Required. 2 to 30 characters and can only contain a-z, 0-9, and the dash (-)', unique=True, max_length=30, db_index=True, validators=[django.core.validators.MinLengthValidator(2), django.core.validators.RegexValidator(re.compile('^[0-9a-z-]+$'), 'Enter a valid slug.', 'invalid')]),
        ),
    ]

Здесь -официальное руководство Django по миграции уникальных полей.

Migrations that add unique fields
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 Applying a "plain" migration that adds a unique non-nullable field to a table
 with existing rows will raise an error because the value used to populate
 existing rows is generated only once, thus breaking the unique constraint.

 Therefore, the following steps should be taken. In this example, we'll add a
 non-nullable :class:`~django.db.models.UUIDField` with a default value. Modify
 the respective field according to your needs.

 * Add the field on your model with ``default=...`` and ``unique=True``
   arguments. In the example, we use ``uuid.uuid4`` for the default.

 * Run the :djadmin:`makemigrations` command.

 * Edit the created migration file.

   The generated migration class should look similar to this::

     class Migration(migrations.Migration):

         dependencies = [
             ('myapp', '0003_auto_20150129_1705'),
         ]

         operations = [
             migrations.AddField(
                 model_name='mymodel',
                 name='uuid',
                 field=models.UUIDField(max_length=32, unique=True, default=uuid.uuid4),
             ),
         ]

   You will need to make three changes:

   * Add a second :class:`~django.db.migrations.operations.AddField` operation
     copied from the generated one and change it to
     :class:`~django.db.migrations.operations.AlterField`.

   * On the first operation (``AddField``), change ``unique=True`` to
     ``null=True`` -- this will create the intermediary null field.

   * Between the two operations, add a
     :class:`~django.db.migrations.operations.RunPython` or
     :class:`~django.db.migrations.operations.RunSQL` operation to generate a
     unique value (UUID in the example) for each existing row.

   The resulting migration should look similar to this::

     # -*- coding: utf-8 -*-
     from __future__ import unicode_literals

     from django.db import migrations, models
     import uuid

     def gen_uuid(apps, schema_editor):
         MyModel = apps.get_model('myapp', 'MyModel')
         for row in MyModel.objects.all():
             row.uuid = uuid.uuid4()
             row.save()

     class Migration(migrations.Migration):

         dependencies = [
             ('myapp', '0003_auto_20150129_1705'),
         ]

         operations = [
             migrations.AddField(
                 model_name='mymodel',
                 name='uuid',
                 field=models.UUIDField(default=uuid.uuid4, null=True),
             ),
             # omit reverse_code=... if you don't want the migration to be reversible.
             migrations.RunPython(gen_uuid, reverse_code=migrations.RunPython.noop),
             migrations.AlterField(
                 model_name='mymodel',
                 name='uuid',
                 field=models.UUIDField(default=uuid.uuid4, unique=True),
             ),
         ]
* Now you can apply the migration as usual with the :djadmin:`migrate` command.

   Note there is a race condition if you allow objects to be created while this
   migration is running. Objects created after the ``AddField`` and before
   ``RunPython`` will have their original ``uuid``’s overwritten.

Вы можете вручную отредактировать файл миграции:

Мне нужно было добавить случайный символ в какое-то поле, поэтому я импортировал random и randint

import random
import string

И изменил значение default на

default=random.choice(string.lowercase)

Это сработало.

Есть способ сделать уникальное значение для каждой строки с юга.

Определить слизняка в models.py как:

class Foo(models.Model):
  slug = models.SlugField(unique=True, default='')
  ....

Создать новую миграцию

Запуск python manage.py schemamigration --auto foo

Откройте новый файл миграции и отредактируйте его:

# Change add_column to this:
db.add_column(u'account_funnel', 'slug',
            self.gf('django.db.models.foo.Foo')(default='', 
                  unique=False,  
                  max_length=50),
            keep_default=False)

# right above this add such python code:
foos = orm['foo.Foo'].objects.all()
for foo in foos:
        foo.slug = slugify(funnel.name)
        foo.save()

# Modify slug as unique field
db.create_unique(u'foo_foo', ['slug'])

ps mark this migration as no_dry_run = True
pss do not forget to import slugify function from django.template.defaultfilters import slugify