rovide a one-off default now (will be set on all existing rows with a null value

You are trying to add a non-nullable field 'name' to contact 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 with a null value for this column)
2.Quit, and let me add a default in models.py

解决方法（一）：
先输入1，再在界面上输入默认值。

解决方法（二）：
先给'name'任意初始值：name = models.CharField(max_length=50, default='abc')
然后执行：python manage.py makemirations
再执行：python manage.py migrate
再将default删去，即执行：name = models.CharField(max_length=50)
执行：python manage.py makemirations
再执行：python manage.py migrate