# Django 1.8.18.dev20170123152445 documentation

# Writing database migrations

This document explains how to structure and write database migrations for different scenarios you might encounter. For introductory material on migrations, see the topic guide.

## Data migrations and multiple databases

When using multiple databases, you may need to figure out whether or not to run a migration against a particular database. For example, you may want to only run a migration on a particular database.

In order to do that you can check the database connection’s alias inside a `RunPython` operation by looking at the `schema_editor.connection.alias` attribute:

    from django.db import migrations

    def forwards(apps, schema_editor):
        if not schema_editor.connection.alias == 'default':
            return
        # Your migration code goes here

    class Migration(migrations.Migration):

        dependencies = [
            # Dependencies to other migrations
        ]

        operations = [
            migrations.RunPython(forwards),
        ]

You can also provide hints that will be passed to the `allow_migrate()` method of database routers as `**hints`:

myapp/dbrouters.py

    class MyRouter(object):

        def allow_migrate(self, db, app_label, model_name=None, **hints):
            if 'target_db' in hints:
                return db == hints['target_db']
            return True

Then, to leverage this in your migrations, do the following:

    from django.db import migrations

    def forwards(apps, schema_editor):
        # Your migration code goes here
        ...

    class Migration(migrations.Migration):

        dependencies = [
            # Dependencies to other migrations
        ]

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

If your `RunPython` or `RunSQL` operation only affects one model, it’s good practice to pass `model_name` as a hint to make it as transparent as possible to the router. This is especially important for reusable and third-party apps.

## 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 `UUIDField` with a default value. Modify the respective field according to your needs.

- Add the field on your model with `default=uuid.uuid4` and `unique=True` arguments (choose an appropriate default for the type of the field you’re adding).

- Run the `makemigrations` command. This should generate a migration with an `AddField` operation.

- Generate two empty migration files for the same app by running `makemigrations myapp --empty` twice. We’ve renamed the migration files to give them meaningful names in the examples below.

- Copy the `AddField` operation from the auto-generated migration (the first of the three new files) to the last migration and change `AddField` to `AlterField`. For example:

  0006_remove_uuid_null.py

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

      from django.db import migrations, models
      import uuid

      class Migration(migrations.Migration):

          dependencies = [
              ('myapp', '0005_populate_uuid_values'),
          ]

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

- Edit the first migration file. The generated migration class should look similar to this:

  0004_add_uuid_field.py

      class Migration(migrations.Migration):

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

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

  Change `unique=True` to `null=True` – this will create the intermediary null field and defer creating the unique constraint until we’ve populated unique values on all the rows.

- In the first empty migration file, add a `RunPython` or `RunSQL` operation to generate a unique value (UUID in the example) for each existing row. For example:

  0005_populate_uuid_values.py

      # -*- 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', '0004_add_uuid_field'),
          ]

          operations = [
              # omit reverse_code=... if you don't want the migration to be reversible.
              migrations.RunPython(gen_uuid, reverse_code=migrations.RunPython.noop),
          ]

- Now you can apply the migrations as usual with the `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.

## Controlling the order of migrations

Django determines the order in which migrations should be applied not by the filename of each migration, but by building a graph using two properties on the `Migration` class: `dependencies` and `run_before`.

If you’ve used the `makemigrations` command you’ve probably already seen `dependencies` in action because auto-created migrations have this defined as part of their creation process.

The `dependencies` property is declared like this:

    from django.db import migrations

    class Migration(migrations.Migration):

        dependencies = [
            ('myapp', '0123_the_previous_migration'),
        ]

Usually this will be enough, but from time to time you may need to ensure that your migration runs before other migrations. This is useful, for example, to make third-party apps’ migrations run after your `AUTH_USER_MODEL` replacement.

To achieve this, place all migrations that should depend on yours in the `run_before` attribute on your `Migration` class:

    class Migration(migrations.Migration):
        ...

        run_before = [
            ('third_party_app', '0001_do_awesome'),
        ]

Prefer using `dependencies` over `run_before` when possible. You should only use `run_before` if it is undesirable or impractical to specify `dependencies` in the migration which you want to run after the one you are writing.