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

问题：在执行python manage.py makemigrations learning_logs时，系统会报错，提示：TypeError: __init__() missing 1 required positional argument: 'on_delete'

(ll_env) c:\WorkSpace\SimpleTest\learning_log>python manage.py makemigrations learning_logs
Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "c:\WorkSpace\SimpleTest\learning_log\ll_env\lib\site-packages\django\core\management\__init__.py", line 371, in execute_from_command_line
    utility.execute()
  File "c:\WorkSpace\SimpleTest\learning_log\ll_env\lib\site-packages\django\core\management\__init__.py", line 347, in execute
    django.setup()
  File "c:\WorkSpace\SimpleTest\learning_log\ll_env\lib\site-packages\django\__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "c:\WorkSpace\SimpleTest\learning_log\ll_env\lib\site-packages\django\apps\registry.py", line 112, in populate
    app_config.import_models()
  File "c:\WorkSpace\SimpleTest\learning_log\ll_env\lib\site-packages\django\apps\config.py", line 198, in import_models
    self.models_module = import_module(models_module_name)
  File "C:\python36\lib\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 "c:\WorkSpace\SimpleTest\learning_log\learning_logs\models.py", line 13, in <module>
    class Entry(models.Model):
  File "c:\WorkSpace\SimpleTest\learning_log\learning_logs\models.py", line 15, in Entry
    topic = models.ForeignKey(Topic)
TypeError: __init__() missing 1 required positional argument: 'on_delete'

代码如下：

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] + "..."

解决办法：修改指定外键的方式

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, on_delete=models.CASCADE)
    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] + "..."