Fixed #23932 -- Added how-to on migrating unique fields.

        operations = [

            migrations.RunPython(forwards, hints={'target_db': 'default'}),

        ]

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.