如果您刚刚升级到 Rails 3.1 并且在升级前一切正常，请检查您的 database.yml 配置.我怀疑配置是否因为升级而恢复到默认值.

If you have just upgraded to Rails 3.1 and everything was working before the upgrade, check your database.yml configuration. I doubt if the configuration has gone back to the defaults because of the upgrade.

如果没有，请遵循 Rajkamal 的建议并确保您运行迁移.

If not, follow Rajkamal's suggestion and make sure you run the migrations.