Django 在更新时引用外键

Question

我之前问过这个问题，它被关闭了，因为我没有提供足够的信息来说明情况。这是编辑。

我正在运行 Django 并遇到了这个问题。我正在尝试在我的 MySQL 表中创建或更新一个条目。但是，其中一个键是外键。 （我知道如何使用外键创建表条目，但我的方法不起作用）根据 Django 参考，推荐的方法是将新表条目设置为 default 值。

我做了什么：

item = {}
for entry in params:
    if entry not in ['_state', 'timestamp', 'rule_case', 'session_key']:
        item[entry] = params[entry]

        if entry == 'code':
            item[entry] = Demo_user_sessions.objects.get(session=session_key)

obj, created = Demo_user_sessions.objects.update_or_create(session=session_key, defaults=item)

我通过查找特殊键并将内容替换为引用表的链接来扩展它。 foreignKey 的那个。

这是我正在尝试制作的条目的模型。

class Demo_user_sessions(models.Model):
    session_key = models.CharField(primary_key=True, max_length=8)
    timestamp = models.DateTimeField(auto_now_add=True)
    code = models.ForeignKey('HS_Code', on_delete=models.CASCADE, null=True)
    # ... some other

    class Meta:
        ordering = ['timestamp']

    def __str__(self):
        return self

这是我当前的错误信息：

Traceback (most recent call last):
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/django/views/generic/base.py", line 71, in view
    return self.dispatch(request, *args, **kwargs)
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/rest_framework/views.py", line 505, in dispatch
    response = self.handle_exception(exc)
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/rest_framework/views.py", line 465, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/rest_framework/views.py", line 476, in raise_uncaught_exception
    raise exc
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/rest_framework/views.py", line 502, in dispatch
    response = handler(request, *args, **kwargs)
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/rest_framework/decorators.py", line 50, in handler
    return func(*args, **kwargs)
  File "/Users/5knnbdwm/Python/mantaray_env/Mantaray/main/views.py", line 111, in api_user_session
    obj, created = Demo_user_sessions.objects.update_or_create(session=params['session_key'], defaults=item)
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/django/db/models/manager.py", line 82, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/django/db/models/query.py", line 584, in update_or_create
    setattr(obj, k, v() if callable(v) else v)
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/django/db/models/fields/related_descriptors.py", line 215, in __set__
    'Cannot assign "%r": "%s.%s" must be a "%s" instance.' % (
  File "/Users/5knnbdwm/Python/mantaray_env/lib/python3.8/site-packages/django/db/models/base.py", line 518, in __repr__
    return '<%s: %s>' % (self.__class__.__name__, self)
TypeError: __str__ returned non-string (type Demo_user_sessions)

我有点明白错误消息的问题。我只是不知道要改变什么或做些什么不同。

Answer 1

所以有两个问题，1）__str__ 必须返回实例的字符串表示。 2) item["code"] 必须是HS_Code 的实例，或者你可以使用item["code_id"] = some_integer_id_representing_the_HS_Code。

比如：

def __str__(self):
    return self.session_key

还有：

if entry == 'code':
    item[entry] = HS_Code.objects.get(some_lookup=some_value)

Answer 2

从

更新您的模型

code = models.ForeignKey('HS_Code', on_delete=models.CASCADE, null=True)

到

code = models.ForeignKey(HS_Code, on_delete=models.CASCADE, null=True)