class AuthRouter:
    A router to control all database operations on models in the
    auth application.
    def db_for_read(self, model, **hints):
        Attempts to read auth models go to auth_db.
        if model._meta.app_label == 'auth':
            return 'auth_db'
        return None
    def db_for_write(self, model, **hints):
        Attempts to write auth models go to auth_db.
        if model._meta.app_label == 'auth':
            return 'auth_db'
        return None
    def allow_relation(self, obj1, obj2, **hints):
        Allow relations if a model in the auth app is involved.
        if obj1._meta.app_label == 'auth' or \
           obj2._meta.app_label == 'auth':
           return True
        return None
    def allow_migrate(self, db, app_label, model_name=None, **hints):
        Make sure the auth app only appears in the 'auth_db'
        database.
        if app_label == 'auth':
            return db == 'auth_db'
        return None
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    def allow_migrate(self, db, app_label, **hints):
        for router in self.routers:
            try:
                method = router.allow_migrate
            except AttributeError:
                # If the router doesn't have a method, skip to the next one.
                continue
            allow = method(db, app_label, **hints)
            if allow is not None:
                return allow
        return True
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def allow_migrate(self, db, app_label, model_name=None, **hints):
    Make sure the auth app only appears in the 'auth_db'
    database.
    if app_label == 'auth':
        return db == 'auth_db'
    return None
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def allow_migrate(self, db, app_label, model_name=None, **hints):
    Make sure the auth app only appears in the 'auth_db'
    database.
    if app_label == 'auth':
        return db == 'auth_db'
    elif db == 'auth_db':  # 若数据库名称为 auth_db 但 app_label 不为 auth 直接返回 False
        return False
    else:
        return None
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$ ./manage.py migrate --database=users
$ ./manage.py migrate --database=customers
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...
    def handle(self, *args, **options):
        self.verbosity = options['verbosity']
        self.interactive = options['interactive']
        # Import the 'management' module within each installed app, to register
        # dispatcher events.
        for app_config in apps.get_app_configs():
            if module_has_submodule(app_config.module, "management"):
                import_module('.management', app_config.name)
        db_routers = [import_string(router)() for router in conf.settings.DATABASE_ROUTERS] # 对所有的 routers 进行 migrate 操作
        for connection in connections.all():
            # Hook for backends needing any database preparation
            connection.prepare_database()
            # Work out which apps have migrations and which do not
            executor = MigrationExecutor(connection, self.migration_progress_callback)
            # Raise an error if any migrations are applied before their dependencies.
            executor.loader.check_consistent_history(connection)
            # Before anything else, see if there's conflicting apps and drop out
            # hard if there are any
            conflicts = executor.loader.detect_conflicts()
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...
def connections_support_transactions():  # 判断是否所有数据库支持事务
    """Return True if all connections support transactions."""
    return all(conn.features.supports_transactions for conn in connections.all())
class TransactionTestCase(SimpleTestCase):
    multi_db = False
    @classmethod
        def _databases_names(cls, include_mirrors=True):
            # If the test case has a multi_db=True flag, act on all databases,
            # including mirrors or not. Otherwise, just on the default DB.
            if cls.multi_db:
                return [
                    alias for alias in connections
                    if include_mirrors or not connections[alias].settings_dict['TEST']['MIRROR']
            else:
                return [DEFAULT_DB_ALIAS]
    def _fixture_teardown(self):
        # Allow TRUNCATE ... CASCADE and don't emit the post_migrate signal
        # when flushing only a subset of the apps
        for db_name in self._databases_names(include_mirrors=False):
            # Flush the database
            inhibit_post_migrate = (
                self.available_apps is not None or
                (   # Inhibit the post_migrate signal when using serialized
                    # rollback to avoid trying to recreate the serialized data.
                    self.serialized_rollback and
                    hasattr(connections[db_name], '_test_serialized_contents')
            call_command('flush', verbosity=0, interactive=False,  # 清空数据库表
                         database=db_name, reset_sequences=False,
                         allow_cascade=self.available_apps is not None,
                         inhibit_post_migrate=inhibit_post_migrate)
class TestCase(TransactionTestCase):
        def _fixture_teardown(self):
            if not connections_support_transactions():  # 判断是否所有数据库支持事务
                return super()._fixture_teardown()
            try:
                for db_name in reversed(self._databases_names()):
                    if self._should_check_constraints(connections[db_name]):
                        connections[db_name].check_constraints()
            finally:
                self._rollback_atomics(self.atomics)
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class TestCase(TransactionTestCase):
    此类修复 Django TestCase 中由于使用了多数据库，但是 multi_db 并未指定多数据库，单元测试依然只是在一个数据库上运行。
    但是源码中的 connections_support_transactions 将所有数据库都包含进来了，导致在同时使用 MangoDB 和 MySQL 数据库时，
    MySQL 数据库无法回滚，清空了所有的初始化数据，导致单元测试无法使用初始化的数据。
    @classmethod
    def _databases_support_transactions(cls):
        return all(
            conn.features.supports_transactions
            for conn in connections.all()
            if conn.alias in cls._databases_names()
    def _fixture_setup(self):
        if not self._databases_support_transactions():
            # If the backend does not support transactions, we should reload
            # class data before each test
            self.setUpTestData()
            return super()._fixture_setup()
        assert not self.reset_sequences, 'reset_sequences cannot be used on TestCase instances'
        self.atomics = self._enter_atomics()
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