Question

如何在Django迁移中捕获异常？

我有一个迁移，由于各种遗留原因，我预计有时会失败。我希望能够捕获该错误并在这种情况下运行一些错误处理代码。

具体来说，我正在重命名一个表，有时目标表已经存在，我想合并旧表和新表的内容，然后删除旧表。

我正在运行Django 1.7（:(），我们计划升级到1.8，但它还没有发生。

我的迁移是：

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import models, migrations


class Migration(migrations.Migration):

    dependencies = [
        ('main', '0007_migration_name'),
    ]

    operations = [
        migrations.AlterModelTable(
            name='table_name',
            table='LegacyTableName',
        ),
    ]

当我运行时，我得到了

Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File ".../django/core/management/__init__.py", line 385, in execute_from_command_line
    utility.execute()
  File ".../django/core/management/__init__.py", line 377, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File ".../django/core/management/base.py", line 288, in run_from_argv
    self.execute(*args, **options.__dict__)
  File ".../django/core/management/base.py", line 338, in execute
    output = self.handle(*args, **options)
  File ".../django/core/management/commands/migrate.py", line 161, in handle
    executor.migrate(targets, plan, fake=options.get("fake", False))
  File ".../django/db/migrations/executor.py", line 68, in migrate
    self.apply_migration(migration, fake=fake)
  File ".../django/db/migrations/executor.py", line 102, in apply_migration
    migration.apply(project_state, schema_editor)
  File ".../django/db/migrations/migration.py", line 108, in apply
    operation.database_forwards(self.app_label, schema_editor, project_state, new_state)
  File ".../django/db/migrations/operations/models.py", line 236, in database_forwards
    new_model._meta.db_table,
  File ".../django/db/backends/schema.py", line 350, in alter_db_table
    "new_table": self.quote_name(new_db_table),
  File ".../django/db/backends/schema.py", line 111, in execute
    cursor.execute(sql, params)
  File ".../django/db/backends/utils.py", line 81, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File ".../django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
  File ".../django/db/utils.py", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File ".../django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
  File ".../django/db/backends/mysql/base.py", line 129, in execute
    return self.cursor.execute(query, args)
  File ".../MySQLdb/cursors.py", line 226, in execute
    self.errorhandler(self, exc, value)
  File ".../MySQLdb/connections.py", line 36, in defaulterrorhandler
    raise errorvalue
django.db.utils.OperationalError: (1050, "Table 'LegacyTableName' already exists")

迁移本身提供的所有内容都是operations列表，the docs中似乎没有可选的错误处理参数。

如何捕获OperationalError，以便运行一些Python来合并表？

Answer 1

尝试在Python中捕获数据库异常的问题在于它们可能不够具体 - 例如，OperationalError可能由于各种原因而出现（其中只有一个是表名已经被更改）。

我建议你不要试图捕获异常，而是编写自己的迁移函数，进行必要的检查/修改。请参阅documentation on RunPython。

这通常是您用于创建数据迁移，运行自定义数据更新和更改以及您需要访问ORM和/或Python代码的任何其他操作的操作。

在您的情况下，您将编写一个函数来检查表是否存在并对这两种情况执行某些操作。

编写这些函数时需要注意一些特定于数据库的问题，例如：

例如，在PostgreSQL上，您应该避免在同一次迁移中组合架构更改和RunPython操作，否则可能会遇到错误。

如何处理Django迁移中的异常？

1 个答案: