2019.02.22Django做迁移模型的时候报错TypeError: __init__() missing 1 required positional argument: 'on_delete'

今天，在用Django做迁移模型时，执行迁移数据库python manage.py makemigrations learning_log时出现错误。原代码为：

from django.db import models
# Create your models here.在这里创建模型

class Topic(models.Model):
    """用户学习的主题"""
    text = models.CharField(max_length = 200)
    date_added = models.DateTimeField(auto_now_add = True)
    def __str__(self):
        """返回模型的字符串表示"""
        return self.text

class Entry(models.Model):
    """学到的有关某个主题的具体知识"""
    topic = models.ForeignKey(Topic)
    text = models.TextField()
    date_added = models.DateTimeField(auto_now_add = True)
    class Meta:
        verbose_name_plural = 'entries'
    def __str__(self):
        """返回模型的字符串表示"""
        return self.text[:50] + "..."

错误信息如下所示：

Traceback (most recent call last):
  File "manage.py", line 15, in 
    execute_from_command_line(sys.argv)
        --snip--

  File "D:\Pythondebug\my_program\learning_log\learning_logs\models.py", line 13, in 
    class Entry(models.Model):
  File "D:\Pythondebug\my_program\learning_log\learning_logs\models.py", line 15, in Entry
    topic = models.ForeignKey(Topic)
TypeError: __init__() missing 1 required positional argument: 'on_delete'

这是因为在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。此时将如下代码做替换修改，则问题解决。

    topic = models.ForeignKey(Topic)


    topic = models.ForeignKey('Topic', on_delete=models.CASCADE)