Django2.x版本在生成数据库表初始化文件报错

1.待创建的表信息

from django.db import models

# Create your models here.
class Book(models.Model):
    name=models.CharField(max_length=20)
    price=models.IntegerField()
    pub_date=models.DateField()
    publish=models.ForeignKey("Publish")

    def __str__(self):
        return self.name


class Publish(models.Model):
    name=models.CharField(max_length=32)
    city=models.CharField(max_length=32)

    def __str__(self):
        return self.name

2.运行makemigrations报错

Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "/root/Django_env/ORM_mutil/venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 371, in execute_from_command_line
    utility.execute()
  File "/root/Django_env/ORM_mutil/venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 347, in execute
    django.setup()
  File "/root/Django_env/ORM_mutil/venv/lib/python3.6/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/root/Django_env/ORM_mutil/venv/lib/python3.6/site-packages/django/apps/registry.py", line 112, in populate
    app_config.import_models()
  File "/root/Django_env/ORM_mutil/venv/lib/python3.6/site-packages/django/apps/config.py", line 198, in import_models
    self.models_module = import_module(models_module_name)
  File "/root/.pyenv/versions/3.6.5/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 "/root/Django_env/ORM_mutil/app01/models.py", line 4, in <module>
    class Book(models.Model):
  File "/root/Django_env/ORM_mutil/app01/models.py", line 8, in Book
    publish=models.ForeignKey("Publish")
TypeError: __init__() missing 1 required positional argument: 'on_delete'
(venv) [root@localhost ORM_mutil]# python manage.py makemigrations
Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "/root/Django_env/ORM_mutil/venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 371, in execute_from_command_line
    utility.execute()
  File "/root/Django_env/ORM_mutil/venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 347, in execute
    django.setup()
  File "/root/Django_env/ORM_mutil/venv/lib/python3.6/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/root/Django_env/ORM_mutil/venv/lib/python3.6/site-packages/django/apps/registry.py", line 112, in populate
    app_config.import_models()
  File "/root/Django_env/ORM_mutil/venv/lib/python3.6/site-packages/django/apps/config.py", line 198, in import_models
    self.models_module = import_module(models_module_name)
  File "/root/.pyenv/versions/3.6.5/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 "/root/Django_env/ORM_mutil/app01/models.py", line 4, in <module>
    class Book(models.Model):
  File "/root/Django_env/ORM_mutil/app01/models.py", line 8, in Book
    publish=models.ForeignKey("Publish")
TypeError: __init__() missing 1 required positional argument: 'on_delete'

3.报错原因

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。

4.解决办法

将第八行的代码改为：
publish=models.ForeignKey("Publish",on_delete=models.CASCADE,)
即在外键值的后面加上 on_delete=models.CASCADE