在低版本的Django中，可以直接写模型外键，不用写后面的冲突代码
转换成Django的时候就会报错

$ python manage.py makemigrations
Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "C:\python3.6\lib\site-packages\django\core\management\__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "C:\python3.6\lib\site-packages\django\core\management\__init__.py", line 357, in execute
    django.setup()
  File "C:\python3.6\lib\site-packages\django\__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "C:\python3.6\lib\site-packages\django\apps\registry.py", line 112, in populate
    app_config.import_models()
  File "C:\python3.6\lib\site-packages\django\apps\config.py", line 198, in import_models
    self.models_module = import_module(models_module_name)
  File "C:\python3.6\lib\importlib\__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 978, in _gcd_import
  File "<frozen importlib._bootstrap>", line 961, in _find_and_load
  File "<frozen importlib._bootstrap>", line 950, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 655, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 205, in _call_with_frames_removed
  File "C:\Users\LY\PycharmProjects\untitled2\test1\booktest\models.py", line 14, in <module>
    class HeroInfo(models.Model):
  File "C:\Users\LY\PycharmProjects\untitled2\test1\booktest\models.py", line 18, in HeroInfo
    hbook = models.ForeignKey('bookinfo')
TypeError: __init__() missing 1 required positional argument: 'on_delete'

解决方法：

    hbook = models.ForeignKey('bookinfo', on_delete=models.CASCADE)

在写模型的外键的时候加上on_delete=models.CASCADE