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.
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 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:
class MyRouter:
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.
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, change AddField to
AlterField, and add imports of uuid and models. For example:
# Generated by Django A.B on YYYY-MM-DD HH:MM
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:
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. Also add an import
of uuid. For example:
# Generated by Django A.B on YYYY-MM-DD HH:MM
from django.db import migrations
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(update_fields=['uuid'])
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.
On databases that support DDL transactions (SQLite and PostgreSQL), migrations
will run inside a transaction by default. For use cases such as performing data
migrations on large tables, you may want to prevent a migration from running in
a transaction by setting the atomic attribute to False:
from django.db import migrations
class Migration(migrations.Migration):
atomic = False
Within such a migration, all operations are run without a transaction. It’s
possible to execute parts of the migration inside a transaction using
atomic() or by passing atomic=True to
RunPython.
Here’s an example of a non-atomic data migration that updates a large table in smaller batches:
import uuid
from django.db import migrations, transaction
def gen_uuid(apps, schema_editor):
MyModel = apps.get_model('myapp', 'MyModel')
while MyModel.objects.filter(uuid__isnull=True).exists():
with transaction.atomic():
for row in MyModel.objects.filter(uuid__isnull=True)[:1000]:
row.uuid = uuid.uuid4()
row.save()
class Migration(migrations.Migration):
atomic = False
operations = [
migrations.RunPython(gen_uuid),
]
The atomic attribute doesn’t have an effect on databases that don’t support
DDL transactions (e.g. MySQL, Oracle). (MySQL’s atomic DDL statement support refers to individual
statements rather than multiple statements wrapped in a transaction that can be
rolled back.)
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.
You can use a data migration to move data from one third-party application to another.
If you plan to remove the old app later, you’ll need to set the dependencies
property based on whether or not the old app is installed. Otherwise, you’ll
have missing dependencies once you uninstall the old app. Similarly, you’ll
need to catch LookupError in the apps.get_model() call that
retrieves models from the old app. This approach allows you to deploy your
project anywhere without first installing and then uninstalling the old app.
Here’s a sample migration:
from django.apps import apps as global_apps
from django.db import migrations
def forwards(apps, schema_editor):
try:
OldModel = apps.get_model('old_app', 'OldModel')
except LookupError:
# The old app isn't installed.
return
NewModel = apps.get_model('new_app', 'NewModel')
NewModel.objects.bulk_create(
NewModel(new_attribute=old_object.old_attribute)
for old_object in OldModel.objects.all()
)
class Migration(migrations.Migration):
operations = [
migrations.RunPython(forwards, migrations.RunPython.noop),
]
dependencies = [
('myapp', '0123_the_previous_migration'),
('new_app', '0001_initial'),
]
if global_apps.is_installed('old_app'):
dependencies.append(('old_app', '0001_initial'))
Also consider what you want to happen when the migration is unapplied. You
could either do nothing (as in the example above) or remove some or all of the
data from the new application. Adjust the second argument of the
RunPython operation accordingly.
If you want to change an unmanaged model (managed=False) to managed, you must remove
managed=False and generate a migration before making other schema-related
changes to the model, since schema changes that appear in the migration that
contains the operation to change Meta.managed may not be applied.
Nov 02, 2020