Django model_to_dict 堆栈溢出崩溃

Question

在我的应用程序中，我为所有模型使用了一个通用的 Base 模型。我遇到了 Campus 和 Hall 模型无法解释的行为：

class Base(models.Model):
    id: int = models.AutoField(primary_key=True, editable=False)

    class Meta:
        abstract = True

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self._initial = self.as_dict

    ...
    @property
    def as_dict(self) -> dict:
        fields = [field.name for field in self._meta.fields]
        res = model_to_dict(self, fields=fields)
        return res
    ...

class Campus(Base):
    name: str = models.CharField(max_length=100)

    class Meta:
        verbose_name_plural = "campuses"

    def __str__(self):
        return self.name

class Hall(Base):
    name: str = models.CharField(max_length=100)
    campus: Campus = models.ForeignKey(Campus, on_delete=models.SET_NULL, null=True)

    def save(self, *args, **kwargs):
        if self.campus is not None and isinstance(self.campus, str):
            self.campus = Campus.objects.get_or_create(name=self.campus)[0]
        super().save(*args, **kwargs)

    def __str__(self):
        return f"{self.name}{f' ({self.campus})' if self.campus else ''}"

在管理控制台上，当我尝试删除某个 Campus 实例时，服务器崩溃了。这是（非常长的）跟踪的一小部分：

Fatal Python error: Cannot recover from stack overflow.
...
Current thread 0x00000bdc (most recent call first):
...
  File "P:\Python\Coursist\academic_helper\models\base.py", line 142 in as_dict
  File "P:\Python\Coursist\academic_helper\models\base.py", line 98 in __init__
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\base.py", line 512 in from_db
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\query.py", line 75 in __iter__
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\query.py", line 1261 in _fetch_all
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\query.py", line 258 in __len__
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\query.py", line 411 in get
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\base.py", line 627 in refresh_from_db
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\query_utils.py", line 139 in __get__
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\fields\__init__.py", line 931 in value_from_object
  File "C:\Venvs\Coursist\lib\site-packages\django\forms\models.py", line 93 in model_to_dict
  File "P:\Python\Coursist\academic_helper\models\base.py", line 142 in as_dict
  File "P:\Python\Coursist\academic_helper\models\base.py", line 98 in __init__
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\base.py", line 512 in from_db
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\query.py", line 75 in __iter__
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\query.py", line 1261 in _fetch_all
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\query.py", line 258 in __len__
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\query.py", line 411 in get
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\base.py", line 627 in refresh_from_db
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\query_utils.py", line 139 in __get__
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\fields\__init__.py", line 931 in value_from_object
  File "C:\Venvs\Coursist\lib\site-packages\django\forms\models.py", line 93 in model_to_dict
  File "P:\Python\Coursist\academic_helper\models\base.py", line 142 in as_dict
  File "P:\Python\Coursist\academic_helper\models\base.py", line 98 in __init__
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\base.py", line 512 in from_db
  File "C:\Venvs\Coursist\lib\site-packages\django\db\models\query.py", line 75 in __iter__
...

起初我认为这是Hall 和Campus 之间的某种循环指向问题，但似乎并非如此（调试时，我看不到Campus 指向的任何指针Hall)。奇怪的是，如果我先删除所有Hall 实例，Campus 的删除会顺利进行。
我正在使用python 3.7 和Django 3.0.6

Answer 1

经过一些调试，我想出了这个补丁：

class Base(models.Model):
    id: int = models.AutoField(primary_key=True, editable=False)
...
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        if models.DEFERRED in args:
            # This means we delete it now, not interested in dict logic
            self._initial = dict()
        else:
            self._initial = self.as_dict
...

不确定这是一个优雅/正确的解决方案，但它解决了这个错误，其他一切似乎都正常。
如果其他人有更好的想法，我很乐意听到。

Answer 2

您的 BaseModel 设计导致了这个问题，因为model_to_dict 将解决 *ToMany 关系并创建可能巨大的列表。管理员创建了自己的内存列表以向您显示您将要删除的所有相关模型，从而放大了这一点。

您基本上是在模型的 init 方法中放置一个序列化的表示形式，您已经将其作为属性。如果这是您缓存值的方式，那么我建议您使用django.utils.functions.cached_property 并将机制更改为“首次访问时缓存”而不是“初始化时缓存”：

from django.utils.functional import cached_property


class Base(models.Model):
    id: int = models.AutoField(primary_key=True, editable=False)

    class Meta:
        abstract = True

    ...
    @cached_property
    def as_dict(self) -> dict:
        fields = [field.name for field in self._meta.fields]
        res = model_to_dict(self, fields=fields)
        return res

    @property
    def _initial(self) -> dict:
        # if you're set on the _initial name
        return self.as_dict