Django | 模型类变更后生成迁移文件，报错:You are trying to add a non-nullable field 'BookName' to BookInfo without a default....

报错:

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:

在模型类中添加了一个非空字段，而该模型类对应的表中，已经存在数据，而这些现存的数据并没有新增字段的值