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Django:带有User的model OneToOneField无法添加默认值

one-to-one database-migration django python

Antoni4040 · 技术社区 · 8 年前

我有一个模型,我想添加一个OneToOneField来保存对象的创建者: 型号.py

creator = models.OneToOneField(User, blank=True,
                               default=User.objects.filter(
                                   username="antoni4040"))

我已经有了一个包含项目的数据库,只希望默认值为admin用户,用户名为“antoni4040”。当我尝试在没有默认字段的情况下进行迁移时,它会要求提供默认值,因此我无法摆脱它。但我在运行makemigrations时得到的是:

Migrations for 'jokes_app':
  0008_joke_creator.py:
    - Add field creator to joke
Traceback (most recent call last):
  File "./manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/home/antoni4040/Documents/Jokes_Website/django-jokes/venv/lib/python3.4/site-packages/django/core/management/__init__.py", line 353, in execute_from_command_line
    utility.execute()
  File "/home/antoni4040/Documents/Jokes_Website/django-jokes/venv/lib/python3.4/site-packages/django/core/management/__init__.py", line 345, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/antoni4040/Documents/Jokes_Website/django-jokes/venv/lib/python3.4/site-packages/django/core/management/base.py", line 348, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/antoni4040/Documents/Jokes_Website/django-jokes/venv/lib/python3.4/site-packages/django/core/management/base.py", line 399, in execute
    output = self.handle(*args, **options)
  File "/home/antoni4040/Documents/Jokes_Website/django-jokes/venv/lib/python3.4/site-packages/django/core/management/commands/makemigrations.py", line 150, in handle
    self.write_migration_files(changes)
  File "/home/antoni4040/Documents/Jokes_Website/django-jokes/venv/lib/python3.4/site-packages/django/core/management/commands/makemigrations.py", line 178, in write_migration_files
    migration_string = writer.as_string()
  File "/home/antoni4040/Documents/Jokes_Website/django-jokes/venv/lib/python3.4/site-packages/django/db/migrations/writer.py", line 167, in as_string
    operation_string, operation_imports = OperationWriter(operation).serialize()
  File "/home/antoni4040/Documents/Jokes_Website/django-jokes/venv/lib/python3.4/site-packages/django/db/migrations/writer.py", line 124, in serialize
    _write(arg_name, arg_value)
  File "/home/antoni4040/Documents/Jokes_Website/django-jokes/venv/lib/python3.4/site-packages/django/db/migrations/writer.py", line 88, in _write
    arg_string, arg_imports = MigrationWriter.serialize(_arg_value)
  File "/home/antoni4040/Documents/Jokes_Website/django-jokes/venv/lib/python3.4/site-packages/django/db/migrations/writer.py", line 433, in serialize
    return cls.serialize_deconstructed(path, args, kwargs)
  File "/home/antoni4040/Documents/Jokes_Website/django-jokes/venv/lib/python3.4/site-packages/django/db/migrations/writer.py", line 318, in serialize_deconstructed
    arg_string, arg_imports = cls.serialize(arg)
  File "/home/antoni4040/Documents/Jokes_Website/django-jokes/venv/lib/python3.4/site-packages/django/db/migrations/writer.py", line 517, in serialize
    item_string, item_imports = cls.serialize(item)
  File "/home/antoni4040/Documents/Jokes_Website/django-jokes/venv/lib/python3.4/site-packages/django/db/migrations/writer.py", line 540, in serialize
    "topics/migrations/#migration-serializing" % (value, get_docs_version())
ValueError: Cannot serialize: <User: antoni4040>
There are some values Django cannot serialize into migration files.
For more, see https://docs.djangoproject.com/en/1.9/topics/migrations/#migration-serializing

我做错了什么?

2 回复 | 直到 8 年前

Alasdair 8 年前

您正在使用查询集 User.objects.filter(username="antoni4040") 。要获取模型实例,可以使用 User.objects.get(username="antoni4040") .

但是,您不应该将的模型实例用作模型字段中的默认值。如果数据库中不存在用户,则不会进行错误处理。事实上,如果 models.py 在运行初始迁移之前加载,然后 User 表甚至不存在,因此查询将给出错误。

设置默认用户的逻辑应该放在视图(或Django admin)中,而不是放在模型文件中。

在admin中,您可以定义一个自定义表单,该表单为 creator 领域

class MyModelForm(forms.ModelForm):
    class Meta:
        model = MyModel

    def __init__(self, *args, **kwargs):
        try:
            user = User.objects.get(username="antoni4040")
            kwargs['initial']['creator'] = user
        except MyModel.DoesNotExist:
            pass
        super(MyModelForm, self).__init__(*args, **kwargs)

然后在管理员中使用该模型表单。

class MyModelAdmin(admin.ModelAdmin):
    form = MyModelForm
    ...

wonton 6 年前

如果你真的想这样做,你可以打开一个shell并获取该用户的id,然后设置默认值。

问题是,你为什么要这样做?第一次添加模型时,它将正确使用默认值。当你第二次尝试这样做时,它会失败,因为它是一对一,而不是多对一,所以它会失败。