Django：带有用户的模型 OneToOneField 无法添加默认值

Question

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

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

我已经有一个包含项目的数据库，只是希望默认值是管理员用户，其用户名为“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，所以我无法摆脱它。但这是我在运行 makemigrations 时得到的结果：

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？

Answer 1

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

但是，您不应该在模型字段中使用模型实例作为默认值。如果用户在数据库中不存在，则没有错误处理。事实上，如果在运行初始迁移之前加载了 models.py，那么 User 表甚至将不存在，因此查询会出错。

设置默认用户的逻辑应该放在视图（或 Django 管理员）而不是模型文件中。

在管理员中，您可以定义一个自定义表单来设置creator 字段的初始值。

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)

然后在您的管理员中使用该模型表单。

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

Answer 2

如果您真的想这样做，您可以打开一个 shell 并获取该用户的 id，然后将默认设置为那个。

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。第二次尝试这样做时，它会失败，因为它是一对一的，而不是多对一的，所以它会失败。