在使用 DJango 框架生成迁移文件的命令（python manage.py makemigrations 应用名）时，可以看到出现如下错误：
(ll_env) D:\workspace\workspace-mengll\learning-log>python manage.py makemigrations learning_logs Traceback (most recent call last): File "D:\workspace\workspace-mengll\learning-log\manage.py", line 22, in main() File "D:\workspace\workspace-mengll\learning-log\manage.py", line 18, in main execute_from_command_line(sys.argv) File "D:\workspace\workspace-mengll\learning-log\ll_env\Lib\site-packages\django\core\management\__init__.py", line 442, in execute_from_command_line utility.execute() File "D:\workspace\workspace-mengll\learning-log\ll_env\Lib\site-packages\django\core\management\__init__.py", line 416, in execute django.setup() File "D:\workspace\workspace-mengll\learning-log\ll_env\Lib\site-packages\django\__init__.py", line 24, in setup apps.populate(settings.INSTALLED_APPS) File "D:\workspace\workspace-mengll\learning-log\ll_env\Lib\site-packages\django\apps\registry.py", line 116, in populate app_config.import_models() File "D:\workspace\workspace-mengll\learning-log\ll_env\Lib\site-packages\django\apps\config.py", line 269, in import_models self.models_module = import_module(models_module_name) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "D:\Python\Python312\Lib\importlib\__init__.py", line 90, in import_module return _bootstrap._gcd_import(name[level:], package, level) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "", line 1387, in _gcd_import File "", line 1360, in _find_and_load File "", line 1331, in _find_and_load_unlocked File "", line 935, in _load_unlocked File "", line 994, in exec_module File "", line 488, in _call_with_frames_removed File "D:\workspace\workspace-mengll\learning-log\learning_logs\models.py", line 15, in class Entry(models.Model): File "D:\workspace\workspace-mengll\learning-log\learning_logs\models.py", line 17, in Entry topic = models.ForeignKey(Topic) ^^^^^^^^^^^^^^^^^^^^^^^^TypeError: ForeignKey.__init__() missing 1 required positional argument: 'on_delete'
(ll_env) D:\workspace\workspace-mengll\learning-log>
错误原因是在 DJango 2.0 后，定义外键和一对一关系的时候需要手动加 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，而在老版本这个参数（models.CASCADE）是默认值。
所以在模型配置外键时，使用新版本的 DJango 时需要明确指定该配置才行，默认是不添加该参数的。修改代码如下：
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)
修改为：
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)
再执行生成迁移文件的命令时，可以看到错误已解决。
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