Django在根据models生成数据库表时报 __init__() missing 1 required positional argument 'on_d

from django.db import models


class Blog(models.Model):
    name = models.CharField(max_length=100)
    tagline = models.TextField()

    def __unicode__(self):  # __str__ on Python 3
        return self.name


class Author(models.Model):
    name = models.CharField(max_length=50)
    email = models.EmailField()

    def __unicode__(self):  # __str__ on Python 3
        return self.name


class Entry(models.Model):
    blog = models.ForeignKey(Blog)
    headline = models.CharField(max_length=255)
    body_text = models.TextField()
    pub_date = models.DateField()
    mod_date = models.DateField()
    authors = models.ManyToManyField(Author)
    n_comments = models.IntegerField()
    n_pingbacks = models.IntegerField()
    rating = models.IntegerField()

    def __unicode__(self):  # __str__ on Python 3
        return self.headline

Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "E:\diango\venv\lib\site-packages\django\core\management\__init__.py", line 371, in execute_from_command_line
    utility.execute()
  File "E:\diango\venv\lib\site-packages\django\core\management\__init__.py", line 347, in execute
    django.setup()
  File "E:\diango\venv\lib\site-packages\django\__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "E:\diango\venv\lib\site-packages\django\apps\registry.py", line 112, in populate
    app_config.import_models()
  File "E:\diango\venv\lib\site-packages\django\apps\config.py", line 198, in import_models
    self.models_module = import_module(models_module_name)
  File "E:\Program Files\Anaconda3\lib\importlib\__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 986, in _gcd_import
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 665, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "E:\diango\blog\models.py", line 24, in <module>
    class Entry(models.Model):
  File "E:\diango\blog\models.py", line 25, in Entry
    blog = models.ForeignKey(Blog)
TypeError: __init__() missing 1 required positional argument: 'on_delete'