我在尝试通过多个 OneToOneField 关系选择相关时遇到一个奇怪的错误,例如,在目标字段是孙子类的情况下。我希望有人能帮助我了解发生了什么(或确认这是 Django 中的一个错误)。
插图:
# models.py
from django.db import models
class A(models.Model):
pass
class B(A):
pass
class C(B):
pass
很简单,对吧?现在我用一个干净的数据库打开 Django shell:
>>> C().save()
>>> A.objects.select_related('b__c')
[]
等等,什么?为什么那个查询集是空的?快速健全性检查:
>>> A.objects.select_related('b')[0].b.c
<C: C object>
那么为什么 select_related 调用不起作用呢?好吧,看这个:
>>> A.objects.select_related('b__c').__iter__().next()
...
Traceback (most recent call last):
File "<console>", line 1, in <module>
File "/opt/webapps/asdf/lib/python2.6/site-packages/django/db/models/query.py", line 107, in _result_iter
self._fill_cache()
File "/opt/webapps/asdf/lib/python2.6/site-packages/django/db/models/query.py", line 772, in _fill_cache
self._result_cache.append(self._iter.next())
File "/opt/webapps/asdf/lib/python2.6/site-packages/django/db/models/query.py", line 273, in iterator
for row in compiler.results_iter():
File "/opt/webapps/asdf/lib/python2.6/site-packages/django/db/models/sql/compiler.py", line 680, in results_iter
for rows in self.execute_sql(MULTI):
File "/opt/webapps/asdf/lib/python2.6/site-packages/django/db/models/sql/compiler.py", line 725, in execute_sql
sql, params = self.as_sql()
File "/opt/webapps/asdf/lib/python2.6/site-packages/django/db/models/sql/compiler.py", line 58, in as_sql
self.pre_sql_setup()
File "/opt/webapps/asdf/lib/python2.6/site-packages/django/db/models/sql/compiler.py", line 29, in pre_sql_setup
self.fill_related_selections()
File "/opt/webapps/asdf/lib/python2.6/site-packages/django/db/models/sql/compiler.py", line 661, in fill_related_selections
used, next, restricted, new_nullable)
File "/opt/webapps/asdf/lib/python2.6/site-packages/django/db/models/sql/compiler.py", line 617, in fill_related_selections
chain = opts.get_base_chain(f.rel.to)
File "/opt/webapps/asdf/lib/python2.6/site-packages/django/db/models/options.py", line 452, in get_base_chain
% model._meta.module_name,)
TypeError: 'b' is not an ancestor of this model
>>>
那么,这是一个 Django 错误,还是我不明白什么?