mirror of
https://github.com/django/django.git
synced 2024-12-27 03:25:58 +00:00
325 lines
11 KiB
Plaintext
325 lines
11 KiB
Plaintext
===========================
|
||
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 :doc:`the topic guide </topics/migrations>`.
|
||
|
||
.. _data-migrations-and-multiple-databases:
|
||
|
||
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 :meth:`allow_migrate()`
|
||
method of database routers as ``**hints``:
|
||
|
||
.. snippet::
|
||
:filename: 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 :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=uuid.uuid4`` and ``unique=True``
|
||
arguments (choose an appropriate default for the type of the field you're
|
||
adding).
|
||
|
||
* Run the :djadmin:`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:
|
||
|
||
.. snippet::
|
||
:filename: 0006_remove_uuid_null.py
|
||
|
||
# -*- coding: utf-8 -*-
|
||
# Generated by Django A.B on YYYY-MM-DD HH:MM
|
||
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:
|
||
|
||
.. snippet::
|
||
:filename: 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
|
||
: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. For example:
|
||
|
||
.. snippet::
|
||
:filename: 0005_populate_uuid_values.py
|
||
|
||
# -*- coding: utf-8 -*-
|
||
# Generated by Django A.B on YYYY-MM-DD HH:MM
|
||
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 :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.
|
||
|
||
.. _non-atomic-migrations:
|
||
|
||
Non-atomic migrations
|
||
~~~~~~~~~~~~~~~~~~~~~
|
||
|
||
.. versionadded:: 1.10
|
||
|
||
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
|
||
:func:`~django.db.transaction.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).
|
||
|
||
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 :djadmin:`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 :setting:`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.
|
||
|
||
Migrating data between third-party apps
|
||
=======================================
|
||
|
||
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 :exc:`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:
|
||
|
||
.. snippet::
|
||
:filename: myapp/migrations/0124_move_old_app_to_new_app.py
|
||
|
||
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
|
||
:mod:`~django.db.migrations.operations.RunPython` operation accordingly.
|