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<p>假设一个Django应用程序应该使用两个MySQL数据库:</p>
<ul>
<li><code>default</code>-用于存储由模型<code>A</code>和<code>B</code>表示的数据(读写访问)</li>
<li><code>support</code>-用于导入由模型<code>C</code>和<code>D</code>表示的数据(只读访问)</li>
</ul>
<p><code>support</code>数据库是外部应用程序的一部分,不能修改。</p>
<p>由于Django应用程序对模型<code>A</code>和<code>B</code>使用内置的ORM,我认为它应该对模型<code>C</code>和<code>D</code>使用相同的ORM,即使它们映射到外部数据库中的表(<code>support</code>)</p>
<p>为了实现这一点,我定义了模型<code>C</code>和<code>D</code>,如下所示:</p>
<pre class="lang-python prettyprint-override"><code>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'
</code></pre>
<p>然后我定义了一个数据库路由器:</p>
<pre class="lang-python prettyprint-override"><code>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')
</code></pre>
<p>最后调整<code>settings.py</code>:</p>
<pre class="lang-python prettyprint-override"><code># (...)
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']
# (...)
</code></pre>
<p>在<code>support.conf</code>中为<code>support</code>数据库指定的用户已被分配只读权限。</p>
<p>但是当我运行<code>python manage.py makemigrations</code>时,它失败了,输出如下:</p>
<pre class="lang-none prettyprint-override"><code> 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'"))
</code></pre>
<p>看起来Django试图在只读数据库中创建<code>django_migrations</code>表。</p>
<p>有什么干净的方法可以防止迁移机制尝试这样做吗?或者我必须使用另一个ORM库来对<code>support</code>数据库进行只读访问吗?</p>