Django | 模型類變更後生成遷移文件，報錯:You are trying to add a non-nullable field 'BookName' to BookInfo without a default....

報錯:

You are trying to add a non-nullable field 'BookName' to BookInfo without a default; we can't do that (the database needs something to populate existing rows).

Please select a fix:

 1) Provide a one-off default now (will be set on all existing rows)

 2) Quit, and let me add a default in models.py

原因:

在模型類中添加了一個非空欄位，而該模型類對應的表中，已經存在數據，而這些現存的數據並沒有新增欄位的值