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我在使用django-polymorphic的 Django 项目（托管在 Heroku 上）中有这个模型设置：

class Product(PolymorphicModel):
    ...

class Book(Product):
    ...

问题是，我设法创建了 aProduct和 a的损坏实例Book。当我有一个实例Product，我想将其转换为一个实例时，就会发生这种情况，Book并且在精神错乱的时刻做了这样一件愚蠢的事情：

> p
<Product: SomeTitle>
> b = Book()
> b.__dict__ = p.__dict__
> b.save()

现在，我知道这很愚蠢，并且引发了各种错误。我的问题是，现在我遇到了一个p无法删除的实例。调用时p.delete()出现以下错误：

TypeError                                 Traceback (most recent call last)
<ipython-input-3-6a6f0dca8e5f> in <module>()
----> 1 p.delete()

~/.heroku/python/lib/python3.6/site-packages/django/db/models/base.py in delete(self, using, keep_parents)
    971
    972         collector = Collector(using=using)
--> 973         collector.collect([self], keep_parents=keep_parents)
    974         return collector.delete()
    975

~/.heroku/python/lib/python3.6/site-packages/django/db/models/deletion.py in collect(self, objs, source, nullable, collect_related, source_attr, reverse_dependency, keep_parents)
    220                     if self.can_fast_delete(sub_objs, from_field=field):
    221                         self.fast_deletes.append(sub_objs)
--> 222                     elif sub_objs:
    223                         field.remote_field.on_delete(self, field, sub_objs, self.using)
    224             for field in model._meta.private_fields:

~/.heroku/python/lib/python3.6/site-packages/django/db/models/query.py in __bool__(self)
    252
    253     def __bool__(self):
--> 254         self._fetch_all()
    255         return bool(self._result_cache)
    256

~/.heroku/python/lib/python3.6/site-packages/django/db/models/query.py in _fetch_all(self)
   1116     def _fetch_all(self):
   1117         if self._result_cache is None:
-> 1118             self._result_cache = list(self._iterable_class(self))
   1119         if self._prefetch_related_lookups and not self._prefetch_done:
   1120             self._prefetch_related_objects()

~/.heroku/python/lib/python3.6/site-packages/polymorphic/query.py in _polymorphic_iterator(self, base_iter)
     60                     break
     61
---> 62             real_results = self.queryset._get_real_instances(base_result_objects)
     63
     64             for o in real_results:

~/.heroku/python/lib/python3.6/site-packages/polymorphic/query.py in _get_real_instances(self, base_result_objects)
    360
    361                 else:
--> 362                     real_concrete_class = base_object.get_real_instance_class()
    363                     real_concrete_class_id = base_object.get_real_concrete_instance_class_id()
    364

~/.heroku/python/lib/python3.6/site-packages/polymorphic/models.py in get_real_instance_class(self)
    110         if model is not None \
    111                 and not issubclass(model, self.__class__) \
--> 112                 and not issubclass(model, self.__class__._meta.proxy_for_model):
    113             raise PolymorphicTypeInvalid("ContentType {0} for {1} #{2} does not point to a subclass!".format(
    114                 self.polymorphic_ctype_id, model, self.pk,

TypeError: issubclass() arg 2 must be a class or tuple of classes

此外，如果我尝试做

> p = Product.objects.get(title='SomeTitle')
> p
<Product: SomeTitle>
> b = Book.objects.get(title=p.title)

我犯了同样的错误。如果我创建包含此Book实例的任何 QuerySet，它也会出现，这是一个问题。

我的问题是：如何规避此问题并删除损坏的实例？

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