[Fixed]-Pass South random unique default values when migrating

In django 1.7+ you can do the following. It first adds the field with no indexing and no unique. It then assigns the unique values (I based them on the name and used slugify method which you need to create) and finally alters the field again to add index and unique attributes.

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')]),
        ),
    ]

Here is the Django’s official how-to on migrating unique fields.

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.

You can manually edit your migration file:

I needed to add random character to some field so I have imported random and randint

import random
import string

and changed the value of default to

default=random.choice(string.lowercase)

It worked.

There is way to do unique value for each row with South.

Define slug in models.py as:

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

Create new migration

run python manage.py schemamigration –auto foo

Open new migration file, and edit it:

# 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