django 数据库报错

django用数据库当缓存：

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.db.DatabaseCache',
        'LOCATION': 'cache_table',  # 数据库表
        'OPTIONS': {
            'MAX_ENTRIES': 300,  # 最大缓存记录的数量（默认300）
            'CULL_FREQUENCY': 3,  # 缓存到达最大个数之后，剔除缓存个数的比例，即：1/CULL_FREQUENCY（默认3）
        }
    }
}

　　

创建缓存表

python manage.py createcachetable  --traceback


  File "E:\work\coding\LottoryMatch\venv\lib\site-packages\django\core\management\commands\createcachetable.py", line 43, in handle
    self.create_table(db, cache._table, dry_run)
  File "E:\work\coding\LottoryMatch\venv\lib\site-packages\django\core\management\commands\createcachetable.py", line 103, in create_table
    (tablename, e))
django.core.management.base.CommandError: Cache table 'cache_table' could not be created.
The error was: (1071, 'Specified key was too long; max key length is 767 bytes').

　　

带引创建表的信息

print(full_statement)

CREATE TABLE `cache_table` (
    `cache_key` varchar(255) NOT NULL PRIMARY KEY,
    `value` longtext NOT NULL,
    `expires` datetime(6) NOT NULL
);

　　

 修改varchar 767/4->191

        with transaction.atomic(using=database, savepoint=connection.features.can_rollback_ddl):
            with connection.cursor() as curs:
                try:
                    print(full_statement)
                    if '''`cache_key` varchar(255)''' in full_statement:
                        full_statement = full_statement.replace('`cache_key` varchar(255)', '`cache_key` varchar(191)')

                    curs.execute(full_statement)
                except DatabaseError as e:
                    raise CommandError(
                        "Cache table '%s' could not be created.\nThe error was: %s." %
                        (tablename, e))
                for statement in index_output:
                    curs.execute(statement)