django 中出现TypeError: __init__() missing 1 required positional argument: 'on_delete'

错误：

C:UsersAdministratorDesktopcode_releaseappsmodels.py changed, reloading.
Watching for file changes with StatReloader
Exception in thread django-main-thread:
Traceback (most recent call last):
  File "D:软件Python 3.6.4lib	hreading.py", line 916, in _bootstrap_inner
    self.run()
  File "D:软件Python 3.6.4lib	hreading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "D:软件Python 3.6.4libsite-packagesdjangoutilsautoreload.py", line 53, in wrapper
    fn(*args, **kwargs)
  File "D:软件Python 3.6.4libsite-packagesdjangocoremanagementcommands
unserver.py", line 109, in inner_run
    autoreload.raise_last_exception()
  File "D:软件Python 3.6.4libsite-packagesdjangoutilsautoreload.py", line 76, in raise_last_exception
    raise _exception[1]
  File "D:软件Python 3.6.4libsite-packagesdjangocoremanagement\__init__.py", line 357, in execute
    autoreload.check_errors(django.setup)()
  File "D:软件Python 3.6.4libsite-packagesdjangoutilsautoreload.py", line 53, in wrapper
    fn(*args, **kwargs)
  File "D:软件Python 3.6.4libsite-packagesdjango\__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "D:软件Python 3.6.4libsite-packagesdjangoapps
egistry.py", line 114, in populate
    app_config.import_models()
  File "D:软件Python 3.6.4libsite-packagesdjangoappsconfig.py", line 211, in import_models
    self.models_module = import_module(models_module_name)
  File "D:软件Python 3.6.4libimportlib\__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 "C:UsersAdministratorDesktopcode_releaseappsmodels.py", line 36, in <module>
    class DeployTask(models.Model):
  File "C:UsersAdministratorDesktopcode_releaseappsmodels.py", line 39, in DeployTask
    project = models.ForeignKey(verbose_name='项目环境', to='Project')
TypeError: __init__() missing 1 required positional argument: 'on_delete'

解决：

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'

   project = models.ForeignKey(verbose_name='项目环境', to='Project', on_delete=models.CASCADE)