Question

好的，所以我在official documentation上关注Django 2.0教程当我意识到我将模型命名为“问题”而不是“问题”时。（我是Django的新手）

我已经做过

$ python manage.py makemigrations polls
$ python manage.py migrate

所以我想我可以重复一遍以应用更改。

Django问我是否要重命名，所以我同意。

(venv) H:\PycharmProjects\django_tutorials\mysite>python manage.py makemigrations
Did you rename the polls.Questions model to Question? [y/N] y
Migrations for 'polls':
  polls\migrations\0002_auto_20180804_0935.py
    - Rename model Questions to Question

但是当我尝试迁移时，Django不会运行迁移并向我显示此错误。

(venv) H:\PycharmProjects\django_tutorials\mysite>python manage.py migrate
Operations to perform:
  Apply all migrations: admin, auth, contenttypes, polls, sessions
Running migrations:
  Applying polls.0001_initial... OK
  Applying polls.0002_auto_20180804_0935...Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "H:\PycharmProjects\django_tutorials\venv\lib\site-packages\django\core\management\__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "H:\PycharmProjects\django_tutorials\venv\lib\site-packages\django\core\management\__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "H:\PycharmProjects\django_tutorials\venv\lib\site-packages\django\core\management\base.py", line 316, in run_from_argv
    self.execute(*args, **cmd_options)
  File "H:\PycharmProjects\django_tutorials\venv\lib\site-packages\django\core\management\base.py", line 353, in execute
    output = self.handle(*args, **options)
  File "H:\PycharmProjects\django_tutorials\venv\lib\site-packages\django\core\management\base.py", line 83, in wrapped
    res = handle_func(*args, **kwargs)
  File "H:\PycharmProjects\django_tutorials\venv\lib\site-packages\django\core\management\commands\migrate.py", line 203, in handle
    fake_initial=fake_initial,
  File "H:\PycharmProjects\django_tutorials\venv\lib\site-packages\django\db\migrations\executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "H:\PycharmProjects\django_tutorials\venv\lib\site-packages\django\db\migrations\executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "H:\PycharmProjects\django_tutorials\venv\lib\site-packages\django\db\migrations\executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File "H:\PycharmProjects\django_tutorials\venv\lib\site-packages\django\db\migrations\migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "H:\PycharmProjects\django_tutorials\venv\lib\site-packages\django\db\migrations\operations\models.py", line 330, in database_forwards
    new_model._meta.db_table,
  File "H:\PycharmProjects\django_tutorials\venv\lib\site-packages\django\db\backends\sqlite3\schema.py", line 84, in alter_db_table
    ) % old_db_table)
django.db.utils.NotSupportedError: Renaming the 'polls_questions' table while in a transaction is not supported on SQLite because it would break referential integrity. Try adding `atomic = False` to the Migration class.

我用Google搜索，one Stackoverflow question说我应该添加atomic=False，但他从未解释原因。

我还在official documentation上搜索了“非原子迁移”，但仍然看不到为什么需要简单的模型名称更改。（说实话，很难理解我的意思。）

真的应该这样来更改型号名称吗？我做错了什么吗？

非原子迁移

在支持DDL事务的数据库上   （SQLite和PostgreSQL），迁移将通过   默认。对于诸如在大型计算机上执行数据迁移的用例   表中，您可能要阻止迁移在   通过将atomic属性设置为False进行交易：

Answer 1

TL; DR

您可以在SQLite上set atomic=False或use PostgreSQL, MySQL进行原子表重命名。

详细信息

也许您已经知道，此错误在django\db\backends\sqlite3\schema.py", line 84, in alter_db_table处出现

如果您想知道为什么可以看到此代码。（这将覆盖base/schema.py）https://github.com/django/django/blob/master/django/db/backends/sqlite3/schema.py#L77

def alter_db_table(self, model, old_db_table, new_db_table, disable_constraints=True):
    if disable_constraints and self._is_referenced_by_fk_constraint(old_db_table):
        if self.connection.in_atomic_block:
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ # if checked atomic, raise NotSupportedError
            raise NotSupportedError((
                'Renaming the %r table while in a transaction is not '
                'supported on SQLite because it would break referential '
                'integrity. Try adding `atomic = False` to the Migration class.'
            ) % old_db_table)
        self.connection.enable_constraint_checking()
        super().alter_db_table(model, old_db_table, new_db_table)
        self.connection.disable_constraint_checking()
    else:
        super().alter_db_table(model, old_db_table, new_db_table)

然后您可以比较默认的Postgresql或Mysql（它们不会覆盖base/schema.py）https://github.com/django/django/blob/master/django/db/backends/base/schema.py#L399

def alter_db_table(self, model, old_db_table, new_db_table):
    """Rename the table a model points to."""
    if (old_db_table == new_db_table or
        (self.connection.features.ignores_table_name_case and
            old_db_table.lower() == new_db_table.lower())):
        return
    self.execute(self.sql_rename_table % {
        "old_table": self.quote_name(old_db_table),
        "new_table": self.quote_name(new_db_table),
    })
    # Rename all references to the old table name.
    for sql in self.deferred_sql:
        if isinstance(sql, Statement):
            sql.rename_table_references(old_db_table, new_db_table)

摘要
Django的Sqlite3不允许原子选项重命名表。
您可以使用Django的Postgresql，Mysql等（允许原子表重命名）。

Answer 2

简短答案：

是的，这是更改型号名称的方法。您做对了，就像您提到的non-atomic migrations中所述。

详细答案：

来自SQLite Tutorial：

SQLite是一个事务数据库，所有更改和查询都可以原子的，一致的，隔离的和持久的（ACID）。

Atomic表示当您提交事务时，整个交易是否适用。

默认情况下，SQLite在自动提交模式下运行。这意味着每个命令，SQLite都会启动，处理和提交事务自动。

现在假设您的数据库中有几个表，您决定重命名其中一个表。 SQLite自动启动事务，并在该事务中尝试重命名表。但是，例如，您的表被其他表引用，因此引发了异常（如您从@EunChong Lee先前提供的代码中可以看到的那样。）

if self.connection.in_atomic_block:
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ # if checked atomic, raise NotSupportedError
        raise NotSupportedError((
            'Renaming the %r table while in a transaction is not '
            'supported on SQLite because it would break referential '
            'integrity. Try adding `atomic = False` to the Migration class.'
        ) % old_db_table)

因此，当您在迁移文件中使用行atomic = False时：

from django.db import migrations

class Migration(migrations.Migration):
    atomic = False

您可以跳过此异常，然后继续重命名表，是的，对我来说看起来并不那么好，但是很好。

幕后发生的事情对我来说也不是很清楚，我想人们应该更多地研究django\db\backends\sqlite3\schema.py中的代码，以更深入地了解如何用atomic=False固定引用以及在什么情况下进行固定。在实施更改时，我们应该更加小心。

在Django中更改模型名称时，为什么必须声明“ atomic = False”？

2 个答案: