1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
2) Quit, and let me add a default in models.py
Select an option:
book新添加一个字段author,然后执行python manage.py makemigrations 之后出现这个，选项一表示添加默认值，选项二退出，此时选1，输入默认值
。加上引号。
然后python manege.py migrate