执行 python manage.py makemigrations 提示错误

错误提示及解决办法

错误提示

执行 python manage.py makemigrations后，出现如下错误提示：

Traceback (most recent call last):
File "manage.py", line 23, in 
main()
File "manage.py", line 19, in main
execute_from_command_line(sys.argv)
File "D:\codeProgram\python3.7\lib\site-packages\django\core\management__init__.py", line 381, in execute_from_command_line
utility.execute()
File "D:\codeProgram\python3.7\lib\site-packages\django\core\management__init__.py", line 357, in execute
django.setup()
File "D:\codeProgram\python3.7\lib\site-packages\django__init__.py", line 24, in setup
apps.populate(settings.INSTALLED_APPS)
File "D:\codeProgram\python3.7\lib\site-packages\django\apps\registry.py", line 114, in populate
app_config.import_models()
File "D:\codeProgram\python3.7\lib\site-packages\django\apps\config.py", line 211, in import_models
self.models_module = import_module(models_module_name)
File "D:\codeProgram\python3.7\lib\importlib__init__.py", line 127, in import_module
return bootstrap._gcd_import(name[level:], package, level)
File "", line 1006, in _gcd_import
File "", line 983, in _find_and_load
File "", line 967, in _find_and_load_unlocked
File "", line 677, in _load_unlocked
File "", line 728, in exec_module
File "", line 219, in _call_with_frames_removed
File "D:\codeStudy\typeidea\typeidea\blog\models.py", line 8, in 
class Category(models.Model):
File "D:\codeStudy\typeidea\typeidea\blog\models.py", line 20, in Category
owner = models.ForeignKey(User, verbose_name="作者")
TypeError: __init_() missing 1 required positional argument: 'on_delete'：

源程序

-- coding: utf-8 --

from __future__ import unicode_literals

from django.contrib.auth.models import User
from django.db import models


class Category(models.Model):
    STATUS_NORMAL = 1
    STATUS_DELETE = 0
    STATUS_ITEMS = (
        (STATUS_NORMAL, '正常'),
        (STATUS_DELETE, '删除'),
    )

    name = models.CharField(max_length=50, verbose_name="名称")
    status = models.PositiveIntegerField(
        default=STATUS_NORMAL, choices=STATUS_ITEMS, verbose_name="状态")
    is_nav = models.BooleanField(default=False, verbose_name="是否为导航")
    owner = models.ForeignKey(User, verbose_name="作者")
    created_time = models.DateTimeField(auto_now_add=True, verbose_name="创建时间")

    class Meta:
        verbose_name = verbose_name_plural = '分类'

解决方法

找了许久，最终问题锁定在：

owner = models.ForeignKey(User, verbose_name="作者")

由于该字段采取了调用django自带User模型，所以采用了外键的方式，经检查，发现是函数的调用参数少了1个，改为：

owner = models.ForeignKey(User, on_delete=None, verbose_name="作者")

即可。
算是一个小错误，显示基础知识的不牢了！
这里整理一下on_delete参数的各个值的含义:

on_delete=None,               # 删除关联表中的数据时,当前表与其关联的field的行为
on_delete=models.CASCADE,     # 删除关联数据,与之关联也删除
on_delete=models.DO_NOTHING,  # 删除关联数据,什么也不做
on_delete=models.PROTECT,     # 删除关联数据,引发错误ProtectedError
models.ForeignKey('关联表', on_delete=models.SET_NULL, blank=True, null=True)
on_delete=models.SET_NULL,    # 删除关联数据,与之关联的值设置为null（前提FK字段需要设置为可空,一对一同理）
 models.ForeignKey('关联表', on_delete=models.SET_DEFAULT, default='默认值')
on_delete=models.SET_DEFAULT, # 删除关联数据,与之关联的值设置为默认值（前提FK字段需要设置默认值,一对一同理）
on_delete=models.SET,         # 删除关联数据,
 a. 与之关联的值设置为指定值,设置：models.SET(值)
 b. 与之关联的值设置为可执行对象的返回值,设置：models.SET(可执行对象)