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

初学Django时，修改models.py后，执行 python manage.py makemigrations learning_logs报错：

  File "D:\python_work\learning_log\learning_logs\models.py", line 14, in <modul
    class Entry(models.Model):
  File "D:\python_work\learning_log\learning_logs\models.py", line 16, in Entry
    topic = models.ForeignKey(Topic)
TypeError: __init__() missing 1 required positional argument: 'on_delete'

源代码：

from django.db import models

# Create your models here.

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] + "..."

问题原因：

django 升级到2.0之后,表与表之间关联的时候,必须要写on_delete参数,否则会报异常:
TypeError: init() missing 1 required positional argument: ‘on_delete’

解决办法：

定义外键的时候需要加上 on_delete=;
即： 

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