bug one：
You are trying to add a non-nullable field 'height' to person 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
问题描述：当数据库表已经建立完成之后，再往models.py该表格所对应的类中再次添加新的字段，然后执行python manage.py makemigrations时，就会产生该bug。
具体原因：当我们创建表完成后，Django会默认觉得我们已经往表中添加了数据。当添加了一个新的字段的时候，表中已经存在的记录也都同时多了这个新添加的字段，但是赋给这个字段什么值Django不知道怎么办了，所以我们需要给这个新字段添加一个默认值。
解决办法：
1.
You are trying to add a non-nullable field 'height' to person 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
Select an option: 2 这里选择2
2.刚才新添加的字段给个默认值。default='默认值'。