Question

假设一个应该使用两个MySQL数据库的Django应用程序：

default - 用于存储由模型A和B表示的数据（读写访问权限）
support - 用于导入模型C和D（只读访问权限）所代表的数据

support数据库是外部应用程序的一部分，无法进行修改。

由于Django应用程序对模型A和B使用了内置ORM，我认为它应该对模型C和D使用相同的ORM，甚至虽然它们映射到外部数据库中的表（support。）

为了实现这一目标，我按如下方式定义了模型C和D：

from django.db import models


class ExternalModel(models.Model):
    class Meta:
        managed = False
        abstract = True


class ModelC(ExternalModel):
    some_field = models.TextField(db_column='some_field')

    class Meta(ExternalModel.Meta):
        db_table = 'some_table_c'


class ModelD(ExternalModel):
    some_other_field = models.TextField(db_column='some_other_field')

    class Meta(ExternalModel.Meta):
        db_table = 'some_table_d'

然后我定义了一个数据库路由器：

from myapp.myapp.models import ExternalModel


class DatabaseRouter(object):
    def db_for_read(self, model, **hints):
        if issubclass(model, ExternalModel):
            return 'support'

        return 'default'

    def db_for_write(self, model, **hints):
        if issubclass(model, ExternalModel):
            return None

        return 'default'

    def allow_relation(self, obj1, obj2, **hints):
        return (isinstance(obj1, ExternalModel) == isinstance(obj2, ExternalModel))

    def allow_migrate(self, db, app_label, model_name=None, **hints):
        return (db == 'default')

最后调整了settings.py：

# (...)

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.mysql',
        'OPTIONS': {
            'read_default_file': os.path.join(BASE_DIR, 'resources', 'default.cnf'),
        },
    },
    'support': {
        'ENGINE': 'django.db.backends.mysql',
        'OPTIONS': {
            'read_default_file': os.path.join(BASE_DIR, 'resources', 'support.cnf'),
        },
    },
}

DATABASE_ROUTERS = ['myapp.database_router.DatabaseRouter']

# (...)

support.conf数据库中为support数据库指定的用户已被分配了只读权限。

但是当我运行python manage.py makemigrations时，它失败并显示以下输出：

    Traceback (most recent call last):
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/db/backends/utils.py", line 62, in execute
    return self.cursor.execute(sql)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/db/backends/mysql/base.py", line 112, in execute
    return self.cursor.execute(query, args)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/MySQLdb/cursors.py", line 226, in execute
    self.errorhandler(self, exc, value)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
    raise errorvalue
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/MySQLdb/cursors.py", line 217, in execute
    res = self._query(query)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/MySQLdb/cursors.py", line 378, in _query
    rowcount = self._do_query(q)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/MySQLdb/cursors.py", line 341, in _do_query
    db.query(q)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/MySQLdb/connections.py", line 280, in query
    _mysql.connection.query(self, query)
_mysql_exceptions.OperationalError: (1142, "CREATE command denied to user 'somedbuser'@'somehost' for table 'django_migrations'")

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/db/migrations/recorder.py", line 57, in ensure_schema
    editor.create_model(self.Migration)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/db/backends/base/schema.py", line 295, in create_model
    self.execute(sql, params or None)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/db/backends/base/schema.py", line 112, in execute
    cursor.execute(sql, params)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/db/backends/utils.py", line 79, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/db/utils.py", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/utils/six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/db/backends/utils.py", line 62, in execute
    return self.cursor.execute(sql)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/db/backends/mysql/base.py", line 112, in execute
    return self.cursor.execute(query, args)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/MySQLdb/cursors.py", line 226, in execute
    self.errorhandler(self, exc, value)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
    raise errorvalue
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/MySQLdb/cursors.py", line 217, in execute
    res = self._query(query)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/MySQLdb/cursors.py", line 378, in _query
    rowcount = self._do_query(q)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/MySQLdb/cursors.py", line 341, in _do_query
    db.query(q)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/MySQLdb/connections.py", line 280, in query
    _mysql.connection.query(self, query)
django.db.utils.OperationalError: (1142, "CREATE command denied to user 'somedbuser'@'somehost' for table 'django_migrations'")

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/core/management/__init__.py", line 367, in execute_from_command_line
    utility.execute()
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/core/management/__init__.py", line 359, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/core/management/base.py", line 305, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/core/management/base.py", line 356, in execute
    output = self.handle(*args, **options)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/core/management/commands/makemigrations.py", line 100, in handle
    loader.check_consistent_history(connection)
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/db/migrations/loader.py", line 276, in check_consistent_history
    applied = recorder.applied_migrations()
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/db/migrations/recorder.py", line 65, in applied_migrations
    self.ensure_schema()
  File "/Users/username/Development/stuff/myapp/lib/python3.5/site-packages/django/db/migrations/recorder.py", line 59, in ensure_schema
    raise MigrationSchemaMissing("Unable to create the django_migrations table (%s)" % exc)
django.db.migrations.exceptions.MigrationSchemaMissing: Unable to create the django_migrations table ((1142, "CREATE command denied to user 'somedbuser'@'somehost' for table 'django_migrations'"))

看来Django试图在只读数据库django_migrations中创建support表。

是否有任何干净的方法来阻止迁移机制尝试？或者我是否必须使用另一个ORM库来进行support数据库的只读访问？

Answer 1

我遇到了同样的问题（使用Django 1.11），这个问题在我的谷歌搜索结果中排名第一。

您的初始解决方案只缺少一个关键部分。你需要告诉Django什么数据库模型'C'和'D'正在使用。什么对我有用：

class ExternalModel(models.Model):
    class Meta:
        managed = False
        abstract = True    
        app_label = 'support'

然后告诉数据库路由器在allow_migrate（）部分遇到app_label时的行为：

    def allow_migrate(self, db, app_label, model_name=None, **hints):
        if app_label == 'support':
            return False
        return (db == 'default')

我不确定这是Django团队眼中最正确的解决方案，但是效果是allow_migrate（）为使用该app_label属性值定义的任何模型返回False。

Django documentation on routers没有明确提到这一点（或者，至少在模型代码示例中明确了ORM如何将'db'的值传递给allow_migrate（）），但在'app_label之间'和'托管'属性，你可以让它工作*。

*在我的情况下，默认为postgres，只读数据库是Oracle 12，通过cx_Oracle。

Answer 2

似乎在Django 1.10.1时间范围内，Tim Graham（主要的Django维护者）接受了一个修补程序来抑制这个特定的异常但后来撤回了补丁，转而支持（大致）以下方法来解决这个问题。使用Django ORM支持只读数据库。

定义一个数据库路由器，如Django documentation on routers中所述我附加了一个示例路由器，路由到基于＆＃39;应用程序的不同数据库模型元中的标志。
在路由器allow_migrations方法中，对任何db参数返回False 对应于只读数据库。这可以防止迁移模型表，无论它们将被路由到何处。
下一部分有点奇怪，但是橡胶撞到了路上实际上回答了原来的问题。保持 makemigrations 尝试在只读中创建 django_migrations 表数据库，不应该路由数据库流量。在示例中路由器，这意味着＆＃39; read_only＆＃39;在DATABASE_APPS_MAPPING中不。
因此，使用＆＃34;使用＆＃34;显式访问只读数据库。（例如，MyReadOnlyModel.objects.using（＆＃39; read_only＆＃39;）。all（）

Django database apps router

Answer 3

有同样的问题。 Django正试图在所有数据库中创建'django_migrations'表。即使没有与只读数据库关联的模型，也会发生这种情况并且所有路由器都指向不同的DB。

我最后也使用了小便。

Django和只读数据库连接

3 个答案: