使用多个数据库不存在 Django 错误关系 "auth_user"
Django error relation "auth_user" does not exist using multiple database
我正在使用多个数据库（同一 postgresql 数据库中的不同模式）开发 Django 1.9 和 python 3.3 项目。当我第一次尝试迁移项目时，出现此错误
Running migrations:
Rendering model states... DONE
Applying contenttypes.0001_initial... OK
Applying auth.0001_initial... OK
Applying MyApp.0001_initial...Traceback (most recent call last):
File "/usr/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
return self.cursor.execute(sql, params)
psycopg2.ProgrammingError: ERROR: relation "auth_user" does not exist
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "manage.py", line 10, in
execute_from_command_line(sys.argv)
File "/usr/lib/python3.4/site-packages/django/core/management/__init__.py", line 350, in execute_from_command_line
utility.execute()
File "/usr/lib/python3.4/site-packages/django/core/management/__init__.py", line 342, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
File "/usr/lib/python3.4/site-packages/django/core/management/base.py", line 348, in run_from_argv
self.execute(*args, **cmd_options)
File "/usr/lib/python3.4/site-packages/django/core/management/base.py", line 399, in execute
output = self.handle(*args, **options)
File "/usr/lib/python3.4/site-packages/django/core/management/commands/migrate.py", line 200, in handle
executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
File "/usr/lib/python3.4/site-packages/django/db/migrations/executor.py", line 92, in migrate
self._migrate_all_forwards(plan, full_plan, fake=fake, fake_initial=fake_initial)
File "/usr/lib/python3.4/site-packages/django/db/migrations/executor.py", line 121, in _migrate_all_forwards
state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
File "/usr/lib/python3.4/site-packages/django/db/migrations/executor.py", line 198, in apply_migration
state = migration.apply(state, schema_editor)
File "/usr/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 90, in __exit__
self.execute(sql)
File "/usr/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 110, in execute
cursor.execute(sql, params)
File "/usr/lib/python3.4/site-packages/django/db/backends/utils.py", line 79, in execute
return super(CursorDebugWrapper, self).execute(sql, params)
File "/usr/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
return self.cursor.execute(sql, params)
File "/usr/lib/python3.4/site-packages/django/db/utils.py", line 95, in __exit__
six.reraise(dj_exc_type, dj_exc_value, traceback)
File "/usr/lib/python3.4/site-packages/django/utils/six.py", line 685, in reraise
raise value.with_traceback(tb)
File "/usr/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: ERROR: relation "auth_user" does not exist
当 auth_user table 未迁移时，此错误似乎出现在其他项目中。在我的例子中，在迁移需要它的应用程序之前使用 manage.py migrate auth 开始我的迁移并不能解决问题。
我怀疑问题出在Django 中使用了不同的数据库。我的 auth_user table 存储在 默认数据库 并且 models.py 的内容被路由到其他数据库.
迁移过程是否在与我的应用程序数据库相同的数据库中查找 auth_user table？是完全不同的东西吗？
问题的答案
其实是跨库引用问题。 Django 无法创建跨数据库外键。
来自Django 1.8 documentation（并且在下一个版本中没有任何解决方案（当前版本是1.10））：
Cross-database relations
Django doesn’t currently provide any support for foreign key or
many-to-many relationships spanning multiple databases. If you have
used a router to partition models to different databases, any foreign
key and many-to-many relationships defined by those models must be
internal to a single database.
This is because of referential integrity. In order to maintain a
relationship between two objects, Django needs to know that the
primary key of the related object is valid. If the primary key is
stored on a separate database, it’s not possible to easily evaluate
the validity of a primary key.
If you’re using Postgres, Oracle, or MySQL with InnoDB, this is
enforced at the database integrity level – database level key
constraints prevent the creation of relations that can’t be validated.
However, if you’re using SQLite or MySQL with MyISAM tables, there is
no enforced referential integrity; as a result, you may be able to
‘fake’ cross database foreign keys. However, this configuration is not
officially supported by Django
如何解决和保持单独的数据库
在我的例子中，因为路由器正在工作，所以有一些技巧可以简化跨数据库对象。
class CrossDBUser(models.Model):
user = models.IntegerField()
def get_user(self):
return User.objects.get(id=self.user)
def set_user(self, user):
self.user = user.id
class MyClassWithCrossDB(CrossDBUser):
field1 = models.CharField(max_length=200, blank=False)
field2 = models.IntegerField(default=0)
有了这个，我可以使用方法 set_user 和 get_user 来处理存储在我的对象 MyClassWithCrossDB.
中的用户
当然它并不完美，因为它不允许像 on_delete=models.CASCADE 这样的自动操作，我不得不使用方法而不是实例变量。但这是解决方法。
我正在使用多个数据库（同一 postgresql 数据库中的不同模式）开发 Django 1.9 和 python 3.3 项目。当我第一次尝试迁移项目时，出现此错误
Running migrations:
Rendering model states... DONE
Applying contenttypes.0001_initial... OK
Applying auth.0001_initial... OK
Applying MyApp.0001_initial...Traceback (most recent call last):
File "/usr/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
return self.cursor.execute(sql, params)
psycopg2.ProgrammingError: ERROR: relation "auth_user" does not exist
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "manage.py", line 10, in
execute_from_command_line(sys.argv)
File "/usr/lib/python3.4/site-packages/django/core/management/__init__.py", line 350, in execute_from_command_line
utility.execute()
File "/usr/lib/python3.4/site-packages/django/core/management/__init__.py", line 342, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
File "/usr/lib/python3.4/site-packages/django/core/management/base.py", line 348, in run_from_argv
self.execute(*args, **cmd_options)
File "/usr/lib/python3.4/site-packages/django/core/management/base.py", line 399, in execute
output = self.handle(*args, **options)
File "/usr/lib/python3.4/site-packages/django/core/management/commands/migrate.py", line 200, in handle
executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
File "/usr/lib/python3.4/site-packages/django/db/migrations/executor.py", line 92, in migrate
self._migrate_all_forwards(plan, full_plan, fake=fake, fake_initial=fake_initial)
File "/usr/lib/python3.4/site-packages/django/db/migrations/executor.py", line 121, in _migrate_all_forwards
state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
File "/usr/lib/python3.4/site-packages/django/db/migrations/executor.py", line 198, in apply_migration
state = migration.apply(state, schema_editor)
File "/usr/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 90, in __exit__
self.execute(sql)
File "/usr/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 110, in execute
cursor.execute(sql, params)
File "/usr/lib/python3.4/site-packages/django/db/backends/utils.py", line 79, in execute
return super(CursorDebugWrapper, self).execute(sql, params)
File "/usr/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
return self.cursor.execute(sql, params)
File "/usr/lib/python3.4/site-packages/django/db/utils.py", line 95, in __exit__
six.reraise(dj_exc_type, dj_exc_value, traceback)
File "/usr/lib/python3.4/site-packages/django/utils/six.py", line 685, in reraise
raise value.with_traceback(tb)
File "/usr/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: ERROR: relation "auth_user" does not exist
当 auth_user table 未迁移时，此错误似乎出现在其他项目中。在我的例子中，在迁移需要它的应用程序之前使用 manage.py migrate auth 开始我的迁移并不能解决问题。
我怀疑问题出在Django 中使用了不同的数据库。我的 auth_user table 存储在 默认数据库 并且 models.py 的内容被路由到其他数据库.
迁移过程是否在与我的应用程序数据库相同的数据库中查找 auth_user table？是完全不同的东西吗？
问题的答案
其实是跨库引用问题。 Django 无法创建跨数据库外键。
来自Django 1.8 documentation（并且在下一个版本中没有任何解决方案（当前版本是1.10））：
Cross-database relations
Django doesn’t currently provide any support for foreign key or many-to-many relationships spanning multiple databases. If you have used a router to partition models to different databases, any foreign key and many-to-many relationships defined by those models must be internal to a single database.
This is because of referential integrity. In order to maintain a relationship between two objects, Django needs to know that the primary key of the related object is valid. If the primary key is stored on a separate database, it’s not possible to easily evaluate the validity of a primary key.
If you’re using Postgres, Oracle, or MySQL with InnoDB, this is enforced at the database integrity level – database level key constraints prevent the creation of relations that can’t be validated.
However, if you’re using SQLite or MySQL with MyISAM tables, there is no enforced referential integrity; as a result, you may be able to ‘fake’ cross database foreign keys. However, this configuration is not officially supported by Django
如何解决和保持单独的数据库
在我的例子中，因为路由器正在工作，所以有一些技巧可以简化跨数据库对象。
class CrossDBUser(models.Model):
user = models.IntegerField()
def get_user(self):
return User.objects.get(id=self.user)
def set_user(self, user):
self.user = user.id
class MyClassWithCrossDB(CrossDBUser):
field1 = models.CharField(max_length=200, blank=False)
field2 = models.IntegerField(default=0)
有了这个，我可以使用方法 set_user 和 get_user 来处理存储在我的对象 MyClassWithCrossDB.
当然它并不完美，因为它不允许像 on_delete=models.CASCADE 这样的自动操作，我不得不使用方法而不是实例变量。但这是解决方法。