Django Migrations Recipe #3 Django Migrations Recipe #3 Django Migrations Recipe #3
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Django, DjangoCon, Migration

Django Migrations Recipe #3

This is the third migration recipe I publish for Django’s migration framework. I published it as part of my talk at DjangoCon Europe 2016 in Budapest. This recipe will show how to rename an app. For simplicity the app must not have any dependencies from other apps!

Take an app “rename_app” with the following 2 models, an “Author” and a “Book”.

# rename_app/models.py

from django.db import models

class Author(models.Model):
    name = models.CharField(max_length=50)

class Book(models.Model):
    title = models.CharField(max_length=50)
    author = models.ForeignKey('rename_app.Author')

Similar to the previous recipe we create the initial migration and a data migration and apply them:

$ python manage.py makemigrations
$ python manage.py makemigrations rename_app --empty --name initial_data

# Edit rename_app/migrations/0002_initial_data.py

$ python manage.py migrate rename_app

The first step in order to rename the app from “rename_app” to “new_app_name” is forcing Django to a particular table name by adding the db_table attribute on the models’ meta classes:

# rename_app/models.py

from django.db import models

class Author(models.Model):
    name = models.CharField(max_length=50)

    class Meta:
        db_table = 'rename_app_author'

class Book(models.Model):
    title = models.CharField(max_length=50)
    author = models.ForeignKey('rename_app.Author')

    class Meta:
        db_table = 'rename_app_book'

Also create the respective migration for this change and apply it:

$ python manage.py makemigrations --name pin_db_tables

# rename_app/migrations/0003_pin_db_tables.py

from django.db import migrations, models
class Migration(migrations.Migration):
    dependencies = [
        ('rename_app', '0002_initial_data'),
    ]
    operations = [
        migrations.AlterModelTable(name='author', table='rename_app_author'),
        migrations.AlterModelTable(name='book', table='rename_app_book'),
    ]

$ python manage.py migrate

The next step is letting Django think you’ve never applied these migrations whilst not changing the database. You do that by passing the string “zero” as a migration name together with the —fake flag to the migrate management command:

$ python manage.py migrate rename_app zero --fake
Operations to perform:
  Unapply all migrations: rename_app
Running migrations:
  Rendering model states... DONE
  Unapplying rename_app.0003_pin_db_tables... FAKED
  Unapplying rename_app.0002_initial_data... FAKED
  Unapplying rename_app.0001_initial... FAKED

After that you need to do the actual rename of the files and references from “rename_app” to “new_app_name” in every file, including your settings, models and migration files.

EXCEPT for the “db_table” attribute! DO NOT CHANGE THIS NOW!

# in settings.py
INSTALLED_APPS = [
    # ...
    'new_app_name.apps.NewAppNameConfig',
]


# in new_app_name/models.py
class Author(models.Model):
    author = models.ForeignKey('new_app_name.Author')

    class Meta:
        db_table = 'rename_app_book'  # Keep as is for now!


# in new_app_name/migrations/0003_pin_db_tables.py and others
dependencies = [
    ('new_app_name', '0002_initial_data'),
]

When you’re done with that and you didn’t make a mistake along the way, let Django know about the new migrations. Again, you need to pass the —fake flag in order to only record the migration as applied and not do any database operations:

$ python manage.py migrate new_app_name --fake
Operations to perform:
  Apply all migrations: new_app_name
Running migrations:
  Rendering model states... DONE
  Applying new_app_name.0001_initial... FAKED
  Applying new_app_name.0002_initial_data... FAKED
  Applying new_app_name.0003_pin_db_tables... FAKED

Last but not least, you can optionally drop the db_table attribute to rename the tables from for example “rename_app_author” to “new_app_name_author”. When you remove the db_table attribute you have to run makemigration again:

# new_app_name/models.py

from django.db import models

class Author(models.Model):
    name = models.CharField(max_length=50)

class Book(models.Model):
    title = models.CharField(max_length=50)
    author = models.ForeignKey('new_app_name.Author')

When you remove the db_table attribute you have to run makemigration again:

What you can see here is, that the table name is reset to None. Django will therefore automatically derive the name from the app name and model name.

$ python manage.py makemigrations --name rename_tables

# new_app_name/migrations/0004_rename_tables.py

from django.db import migrations, models

class Migration(migrations.Migration):
    dependencies = [
        ('new_app_name', '0003_pin_db_tables'),
    ]
    operations = [
        migrations.AlterModelTable(name='author', table=None),
        migrations.AlterModelTable(name='book', table=None),
    ]

$ python manage.py migrate

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