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
問題描述：當數據庫表已經建立完成之后，再往models.py該表格所對應的類中再次添加新的字段，然后執行python manage.py makemigrations時，就會產生該bug。
具體原因：當我們創建表完成后，Django會默認覺得我們已經往表中添加了數據。當添加了一個新的字段的時候，表中已經存在的記錄也都同時多了這個新添加的字段，但是賦給這個字段什么值Django不知道怎么辦了，所以我們需要給這個新字段添加一個默認值。
解決辦法：
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 這里選擇2
2.剛才新添加的字段給個默認值。default='默認值'。