django 2.x创建sql表的外键

运行环境 : django 2.1 

from django.db import models

# Create your models here.

class Grade(models.Model):
    g_name = models.CharField(max_length=32)


class Student(models.Model):
    s_name = models.CharField(max_length=16)
    # s_grade = models.ForeignKey(Grade)  # django version 1.x
    s_grade = models.ForeignKey(to=Grade, on_delete=models.CASCADE) # django version >= 2.x
    """
    关联类名
    on_delete 在2.x版本是必填的
    on_delete= 
        models.CASCADE      级联删除
        PROTECT             保护
        SET(1)              删除后设置为某个值
        SET_DEFAULT         设置为默认值设置为某一个值
        SET_NULL            设置为空
        DO_NOTHING          什么都不变
    """
    # django 2.x的外键设置参数比django 1.x多了一个参数，稍填写第二个参数就会报错
    ############# ERROR ###########
    # book = models.ForeignKey('BookInfo')
    # TypeError: __init__() missing 1 required positional argument: 'on_delete'