Django2.2中数据库迁移TypeError: __init__() missing 1 required positional argument: ‘on_delete‘

models.py中建表代码如下

class Student（models.Model）:
    ……
    grade = models.ForeignKey('grade')

执行数据库迁移语句

python manage.py makemigrations

报错如下：

 File "E:\work\test\Students\myApp\models.py", line 12, in 
    class Student(models.Model):
  File "E:\work\test\Students\myApp\models.py", line 18, in Student
    grade = models.ForeignKey('grade')
TypeError: __init__() missing 1 required positional argument: 'on_delete'

修改如下：

class Student（models.Model）:
    ……
    grade = models.ForeignKey('grade',on_delete=models.CASCADE)

再次执行迁移语句，成功！

(base) E:\work\test\Students>python manage.py makemigrations
Migrations for 'myApp':
  myApp\migrations\0001_initial.py
    - Create model Grade
    - Create model Student

这是因为在django2.0后，定义外键和一对一关系的时候需要加on_delete选项，此参数为了避免两个表里的数据不一致问题，不然会报错。此时将如下代码做替换修改，则问题解决。