Django項目遷移后缺失字段

Question

我有興趣學習 python 和 Django，我已經開始學習這門課程。 我在遷移項目中的模型時遇到困難。 按照課程，在“模型”一章中。

我已按照指示在 boards/models.py 文件中插入給定代碼。 代碼：

from django.db import models
from django.contrib.auth.models import User

class Board(models.Model):
    name = models.CharField(max_length=30, unique=True)
    description = models.CharField(max_length=100)

class Topic(models.Model):
    subject = models.CharField(max_length=255)
    last_updated = models.DateTimeField(auto_now_add=True)
    board = models.ForeignKey(Board, related_name='topics')
    starter = models.ForeignKey(User, related_name='topics')

class Post(models.Model):
    message = models.TextField(max_length=4000)
    topic = models.ForeignKey(Topic, related_name='posts')
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(null=True)
    created_by = models.ForeignKey(User, related_name='posts')
    updated_by = models.ForeignKey(User, null=True, related_name='+')`

按照“遷移模型”一章的說明並運行此命令：“python manage.py makemigrations” 我應該會收到以下消息：

“boards”的遷移：boards/migrations/0001_initial.py - 創建 model Board - 創建 model Post - 創建 model Topic - 添加字段 topic 到 post - 添加字段 updated_by 到 post

相反，我收到以下錯誤：

Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    main()
  File "manage.py", line 18, in main
    execute_from_command_line(sys.argv)
  File "D:\Desktop\myproject\venv\lib\site-packages\django\core\management\__init__.py", line 419, in     execute_from_command_line
    utility.execute()
  File "D:\Desktop\myproject\venv\lib\site-packages\django\core\management\__init__.py", line 395, in     execute
    django.setup()
  File "D:\Desktop\myproject\venv\lib\site-packages\django\__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "D:\Desktop\myproject\venv\lib\site-packages\django\apps\registry.py", line 114, in populate
    app_config.import_models()
  File "D:\Desktop\myproject\venv\lib\site-packages\django\apps\config.py", line 301, in import_models
    self.models_module = import_module(models_module_name)
  File "C:\Users\Zinkov\AppData\Local\Programs\Python37\lib\importlib\__init__.py", line 127, in     import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "D:\Desktop\myproject\myproject\boards\models.py", line 8, in <module>
    class Topic(models.Model):
  File "D:\Desktop\myproject\myproject\boards\models.py", line 11, in Topic
    board = models.ForeignKey(Board, related_name='topics')
TypeError: __init__() missing 1 required positional argument: 'on_delete'`

我在以下幾行中添加了缺少的參數“on_delete”：

board = models.ForeignKey(Board, related_name='topics', on_delete=models.CASCADE)
starter = models.ForeignKey(User, related_name='topics', on_delete=models.CASCADE)

topic = models.ForeignKey(Topic, related_name='posts',max_length=256, on_delete=models.CASCADE)
created_by = models.ForeignKey(User, related_name='posts', on_delete=models.CASCADE)
updated_by = models.ForeignKey(User, null=True, related_name='+',max_length=256, on_delete=models.CASCADE)`

再次運行“makemigrations”命令后，遷移成功，但沒有按預期添加字段。 我的 output 是：

“boards”的遷移：boards\migrations\0001_initial.py - 創建 model Board - 創建 model Topic - 創建 model Post

代替：

“boards”的遷移：boards\migrations\0001_initial.py - 創建 model Board - 創建 model Topic - 創建 model Post - 添加字段 topic 到 post - 添加字段 updated_by 到 post

很抱歉發了這么長的帖子。 謝謝！

Answer 1

在使用 ForeignKey 關系創建 model 時，它接受 on_delete 參數，但該參數丟失了。 添加它應該工作。 請參考以下代碼

class Topic(models.Model):
     subject = models.CharField(max_length=255)
     last_updated = models.DateTimeField(auto_now_add=True)
     board = models.ForeignKey(Board, related_name='topics', on_delete=models.SET_NULL)
     starter = models.ForeignKey(User, related_name='topics', on_delete=models.SET_NULL)

您可以參考此鏈接了解更多 on_delete arguments [https://www.geeksforgeeks.org/python-relational-fields-in-django-models/]