This document explains how to structure and write database migrations for\ndifferent scenarios you might encounter. For introductory material on\nmigrations, see the topic guide.
\nWhen using multiple databases, you may need to figure out whether or not to\nrun a migration against a particular database. For example, you may want to\nonly run a migration on a particular database.
\nIn order to do that you can check the database connection's alias inside a\nRunPython operation by looking at the schema_editor.connection.alias\nattribute:
from django.db import migrations\n\ndef forwards(apps, schema_editor):\n if schema_editor.connection.alias != 'default':\n return\n # Your migration code goes here\n\nclass Migration(migrations.Migration):\n\n dependencies = [\n # Dependencies to other migrations\n ]\n\n operations = [\n migrations.RunPython(forwards),\n ]\nYou can also provide hints that will be passed to the allow_migrate()\nmethod of database routers as **hints:
class MyRouter:\n\n def allow_migrate(self, db, app_label, model_name=None, **hints):\n if 'target_db' in hints:\n return db == hints['target_db']\n return True\nThen, to leverage this in your migrations, do the following:
\nfrom django.db import migrations\n\ndef forwards(apps, schema_editor):\n # Your migration code goes here\n ...\n\nclass Migration(migrations.Migration):\n\n dependencies = [\n # Dependencies to other migrations\n ]\n\n operations = [\n migrations.RunPython(forwards, hints={'target_db': 'default'}),\n ]\nIf your RunPython or RunSQL operation only affects one model, it's good\npractice to pass model_name as a hint to make it as transparent as possible\nto 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\nwith existing rows will raise an error because the value used to populate\nexisting rows is generated only once, thus breaking the unique constraint.
\nTherefore, the following steps should be taken. In this example, we'll add a\nnon-nullable UUIDField with a default value. Modify\nthe respective field according to your needs.
Add the field on your model with default=uuid.uuid4 and unique=True\narguments (choose an appropriate default for the type of the field you're\nadding).
Run the makemigrations command. This should generate a migration\nwith an AddField operation.
Generate two empty migration files for the same app by running\nmakemigrations myapp --empty twice. We've renamed the migration files to\ngive them meaningful names in the examples below.
Copy the AddField operation from the auto-generated migration (the first\nof the three new files) to the last migration, change AddField to\nAlterField, and add imports of uuid and models. For example:
# Generated by Django A.B on YYYY-MM-DD HH:MM\nfrom django.db import migrations, models\nimport uuid\n\nclass Migration(migrations.Migration):\n\n dependencies = [\n ('myapp', '0005_populate_uuid_values'),\n ]\n\n operations = [\n migrations.AlterField(\n model_name='mymodel',\n name='uuid',\n field=models.UUIDField(default=uuid.uuid4, unique=True),\n ),\n ]\nEdit the first migration file. The generated migration class should look\nsimilar to this:
\nclass Migration(migrations.Migration):\n\n dependencies = [\n ('myapp', '0003_auto_20150129_1705'),\n ]\n\n operations = [\n migrations.AddField(\n model_name='mymodel',\n name='uuid',\n field=models.UUIDField(default=uuid.uuid4, unique=True),\n ),\n ]\nChange unique=True to null=True -- this will create the intermediary\nnull field and defer creating the unique constraint until we've populated\nunique values on all the rows.
In the first empty migration file, add a\nRunPython or\nRunSQL operation to generate a\nunique value (UUID in the example) for each existing row. Also add an import\nof uuid. For example:
# Generated by Django A.B on YYYY-MM-DD HH:MM\nfrom django.db import migrations\nimport uuid\n\ndef gen_uuid(apps, schema_editor):\n MyModel = apps.get_model('myapp', 'MyModel')\n for row in MyModel.objects.all():\n row.uuid = uuid.uuid4()\n row.save(update_fields=['uuid'])\n\nclass Migration(migrations.Migration):\n\n dependencies = [\n ('myapp', '0004_add_uuid_field'),\n ]\n\n operations = [\n # omit reverse_code=... if you don't want the migration to be reversible.\n migrations.RunPython(gen_uuid, reverse_code=migrations.RunPython.noop),\n ]\nNow 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\nmigration is running. Objects created after the AddField and before\nRunPython will have their original uuid’s overwritten.
On databases that support DDL transactions (SQLite and PostgreSQL), migrations\nwill run inside a transaction by default. For use cases such as performing data\nmigrations on large tables, you may want to prevent a migration from running in\na transaction by setting the atomic attribute to False:
from django.db import migrations\n\nclass Migration(migrations.Migration):\n atomic = False\nWithin such a migration, all operations are run without a transaction. It's\npossible to execute parts of the migration inside a transaction using\natomic() or by passing atomic=True to\nRunPython.
Here's an example of a non-atomic data migration that updates a large table in\nsmaller batches:
\nimport uuid\n\nfrom django.db import migrations, transaction\n\ndef gen_uuid(apps, schema_editor):\n MyModel = apps.get_model('myapp', 'MyModel')\n while MyModel.objects.filter(uuid__isnull=True).exists():\n with transaction.atomic():\n for row in MyModel.objects.filter(uuid__isnull=True)[:1000]:\n row.uuid = uuid.uuid4()\n row.save()\n\nclass Migration(migrations.Migration):\n atomic = False\n\n operations = [\n migrations.RunPython(gen_uuid),\n ]\nThe atomic attribute doesn't have an effect on databases that don't support\nDDL transactions (e.g. MySQL, Oracle). (MySQL's atomic DDL statement support refers to individual\nstatements rather than multiple statements wrapped in a transaction that can be\nrolled back.)
Django determines the order in which migrations should be applied not by the\nfilename of each migration, but by building a graph using two properties on the\nMigration class: dependencies and run_before.
If you've used the makemigrations command you've probably\nalready seen dependencies in action because auto-created\nmigrations have this defined as part of their creation process.
The dependencies property is declared like this:
from django.db import migrations\n\nclass Migration(migrations.Migration):\n\n dependencies = [\n ('myapp', '0123_the_previous_migration'),\n ]\nUsually this will be enough, but from time to time you may need to\nensure that your migration runs before other migrations. This is\nuseful, for example, to make third-party apps' migrations run after\nyour AUTH_USER_MODEL replacement.
To achieve this, place all migrations that should depend on yours in\nthe run_before attribute on your Migration class:
class Migration(migrations.Migration):\n ...\n\n run_before = [\n ('third_party_app', '0001_do_awesome'),\n ]\nPrefer using dependencies over run_before when possible. You should\nonly use run_before if it is undesirable or impractical to specify\ndependencies in the migration which you want to run after the one you are\nwriting.
你可以使用数据迁移把数据从一个第三方应用程序中转移到另一个。
\n如果你计划要移除旧应用程序，则需要根据是否安装旧应用程序来设置 依赖 属性。否则，一旦你卸载旧应用程序，就会缺失依赖项。同样，你需要在 app.get_model() 捕获 :exec: LookupError 来从旧应用程序中检索模型。这种途径允许你在任何地方部署项目，而无需先安装并且卸载旧应用程序。
\n这是一个迁移示例：
\nfrom django.apps import apps as global_apps\nfrom django.db import migrations\n\ndef forwards(apps, schema_editor):\n try:\n OldModel = apps.get_model('old_app', 'OldModel')\n except LookupError:\n # The old app isn't installed.\n return\n\n NewModel = apps.get_model('new_app', 'NewModel')\n NewModel.objects.bulk_create(\n NewModel(new_attribute=old_object.old_attribute)\n for old_object in OldModel.objects.all()\n )\n\nclass Migration(migrations.Migration):\n operations = [\n migrations.RunPython(forwards, migrations.RunPython.noop),\n ]\n dependencies = [\n ('myapp', '0123_the_previous_migration'),\n ('new_app', '0001_initial'),\n ]\n\n if global_apps.is_installed('old_app'):\n dependencies.append(('old_app', '0001_initial'))\n另外在迁移未执行时，请考虑好什么是你想要发生的。你可以什么都不做（就像上面的示例）或者从新应用中移除一些或全部的数据。相应的调整 RunPython 操作的第二个参数。
如果你想要将非托管模型 (managed=False) 变为托管的，你必须移除 managed=False 并且在对此模型做其他模式相关的改变前生成一次迁移，因为如果迁移中出现模式改变，对 Meta.managed 的修改操作不会被执行。