关于Django迁移出现问题

关于Django迁移出现问题

源码：

#coding:utf-8

from django.db import models

# Create your models here.
class BookInfo(models.Model):
    btitle = models.CharField(max_length=20)
    bpub_date = models.DateTimeField()


class HeroInfo(models.Model):
    hname = models.CharField(max_length=10)
    hgender = models.BooleanField()
    hcontent = models.CharField(max_length=1000)
    # hbook = models.ForeignKey(BookInfo)
    hbook = models.ForeignKey('BookInfo', on_delete=models.CASCADE, )

出错：

(h2) fwl0000@ubuntu:~/桌面/project/test1$ python3 manage.py makemigrations
Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "/home/fwl0000/HOME/.virtualenvs/h2/lib/python3.6/site-packages/django/core/management/__init__.py", line 371, in execute_from_command_line
    utility.execute()
  File "/home/fwl0000/HOME/.virtualenvs/h2/lib/python3.6/site-packages/django/core/management/__init__.py", line 347, in execute
    django.setup()
  File "/home/fwl0000/HOME/.virtualenvs/h2/lib/python3.6/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/home/fwl0000/HOME/.virtualenvs/h2/lib/python3.6/site-packages/django/apps/registry.py", line 112, in populate
    app_config.import_models()
  File "/home/fwl0000/HOME/.virtualenvs/h2/lib/python3.6/site-packages/django/apps/config.py", line 198, in import_models
    self.models_module = import_module(models_module_name)
  File "/home/fwl0000/HOME/.virtualenvs/h2/lib/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 994, in _gcd_import
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load
  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/fwl0000/桌面/project/test1/booktest/models.py", line 11, in <module>
    class HeroInfo(models.Model):
  File "/home/fwl0000/桌面/project/test1/booktest/models.py", line 15, in HeroInfo
    hbook = models.ForeignKey(BookInfo)
TypeError: __init__() missing 1 required positional argument: 'on_delete'

原因是本人安装的Django是2.0.7版本，但由于在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：
TypeError: __init__() missing 1 required positional argument: 'on_delete'
举例说明：
user=models.OneToOneField(User)
owner=models.ForeignKey(UserProfile)
需要改成：

user=models.OneToOneField(User,on_delete=models.CASCADE) --在老版本这个参数（models.CASCADE）是默认值
owner=models.ForeignKey(UserProfile,on_delete=models.CASCADE) --在老版本这个参数（models.CASCADE）是默认值

参数说明：
on_delete有CASCADE、PROTECT、SET_NULL、SET_DEFAULT、SET()五个可选择的值

CASCADE：此值设置，是级联删除。
PROTECT：此值设置，是会报完整性错误。
SET_NULL：此值设置，会把外键设置为null，前提是允许为null。
SET_DEFAULT：此值设置，会把设置为外键的默认值。
SET()：此值设置，会调用外面的值，可以是一个函数。

一般情况下使用CASCADE就可以了。

解决方案：

将第十一行的代码改为：
hbook=models.ForeignKey('BookInfo',on_delete=models.CASCADE,)
即在外键值的后面加上 on_delete=models.CASCADE