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159 lines
5.0 KiB
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159 lines
5.0 KiB
Plaintext
===========================
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Writing database migrations
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===========================
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This document explains how to structure and write database migrations for
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different scenarios you might encounter. For introductory material on
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migrations, see :doc:`the topic guide </topics/migrations>`.
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.. _data-migrations-and-multiple-databases:
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Data migrations and multiple databases
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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When using multiple databases, you may need to figure out whether or not to
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run a migration against a particular database. For example, you may want to
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**only** run a migration on a particular database.
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In order to do that you can check the database connection's alias inside a
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``RunPython`` operation by looking at the ``schema_editor.connection.alias``
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attribute::
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from django.db import migrations
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def forwards(apps, schema_editor):
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if not schema_editor.connection.alias == 'default':
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return
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# Your migration code goes here
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class Migration(migrations.Migration):
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dependencies = [
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# Dependencies to other migrations
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]
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operations = [
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migrations.RunPython(forwards),
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]
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.. versionadded:: 1.8
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You can also provide hints that will be passed to the :meth:`allow_migrate()`
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method of database routers as ``**hints``:
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.. snippet::
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:filename: myapp/dbrouters.py
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class MyRouter(object):
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def allow_migrate(self, db, model, **hints):
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if 'target_db' in hints:
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return db == hints['target_db']
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return True
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Then, to leverage this in your migrations, do the following::
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from django.db import migrations
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def forwards(apps, schema_editor):
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# Your migration code goes here
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class Migration(migrations.Migration):
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dependencies = [
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# Dependencies to other migrations
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]
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operations = [
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migrations.RunPython(forwards, hints={'target_db': 'default'}),
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]
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Migrations that add unique fields
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Applying a "plain" migration that adds a unique non-nullable field to a table
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with existing rows will raise an error because the value used to populate
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existing rows is generated only once, thus breaking the unique constraint.
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Therefore, the following steps should be taken. In this example, we'll add a
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non-nullable :class:`~django.db.models.UUIDField` with a default value. Modify
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the respective field according to your needs.
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* Add the field on your model with ``default=...`` and ``unique=True``
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arguments. In the example, we use ``uuid.uuid4`` for the default.
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* Run the :djadmin:`makemigrations` command.
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* Edit the created migration file.
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The generated migration class should look similar to this::
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class Migration(migrations.Migration):
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dependencies = [
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('myapp', '0003_auto_20150129_1705'),
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]
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operations = [
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migrations.AddField(
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model_name='mymodel',
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name='uuid',
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field=models.UUIDField(max_length=32, unique=True, default=uuid.uuid4),
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),
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]
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You will need to make three changes:
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* Add a second :class:`~django.db.migrations.operations.AddField` operation
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copied from the generated one and change it to
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:class:`~django.db.migrations.operations.AlterField`.
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* On the first operation (``AddField``), change ``unique=True`` to
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``null=True`` -- this will create the intermediary null field.
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* Between the two operations, add a
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:class:`~django.db.migrations.operations.RunPython` or
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:class:`~django.db.migrations.operations.RunSQL` operation to generate a
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unique value (UUID in the example) for each existing row.
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The resulting migration should look similar to this::
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# -*- coding: utf-8 -*-
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from __future__ import unicode_literals
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from django.db import migrations, models
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import uuid
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def gen_uuid(apps, schema_editor):
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MyModel = apps.get_model('myapp', 'MyModel')
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for row in MyModel.objects.all():
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row.uuid = uuid.uuid4()
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row.save()
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class Migration(migrations.Migration):
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dependencies = [
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('myapp', '0003_auto_20150129_1705'),
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]
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operations = [
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migrations.AddField(
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model_name='mymodel',
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name='uuid',
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field=models.UUIDField(default=uuid.uuid4, null=True),
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),
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# omit reverse_code=... if you don't want the migration to be reversible.
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migrations.RunPython(gen_uuid, reverse_code=migrations.RunPython.noop),
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migrations.AlterField(
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model_name='mymodel',
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name='uuid',
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field=models.UUIDField(default=uuid.uuid4, unique=True),
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),
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]
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* Now you can apply the migration as usual with the :djadmin:`migrate` command.
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Note there is a race condition if you allow objects to be created while this
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migration is running. Objects created after the ``AddField`` and before
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``RunPython`` will have their original ``uuid``’s overwritten.
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