在添加、替换或删除一个记录时，这两个值都会被更新。
Both values are updated whenever a record is added, replaced, or deleted.
保持记录，研究你的在线资料，并决定你可以删除什么，下载一个像Freedom这样的程序，使你远离浏览器，或采取 "数字安息日"。
Keep logs to study your online profile and decide what you can knock out, download a program like Freedom that locks you out of your browser, or take a "digital Sabbath".
最后，用一个函数更新MStudent模型来删除此记录。
Finally, update the MStudent model with a function to delete the record.
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