Django select_related with specified break fields for multiple one to one relationships

Question

I get a weird error trying to select_link over multiple OneToOneField relationships, for example. when the target field is a subclass of the grandson. I would like someone to help me understand what is happening (or confirm that this is a bug in Django).

Illustration:

# models.py
from django.db import models

class A(models.Model):
    pass

class B(A):
    pass

class C(B):
    pass

Simple enough, right? Now I open the Django shell with a clean database:

>>> C().save()
>>> A.objects.select_related('b__c')
[]

Wait what? Why is this query set empty? Quick health check:

>>> A.objects.select_related('b')[0].b.c
<C: C object>

So why does select_related call not work? Ok, see this:

>>> 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
>>> 

So is this a Django bug, or am I misunderstanding something?

Answer 1

name = CharField :

>>> a = A(name='a_object')
>>> a.save()
>>> b = B(name='b_object')
>>> b.save()
>>> c = C(name='c_object')
>>> c.save()

>>> A.objects.all()
[<A: A object>, <A: A object>, <A: A object>]
>>> B.objects.all()
[<B: B object>, <B: B object>]
>>> C.objects.all()
[<C: C object>]

>>> A.objects.select_related('b__c')
[]
>>> A.objects.select_related('b__c').__iter__().next()
Traceback ....
    ...
    TypeError: 'b' is not an ancestor of this model

>>> d = A.objects.select_related('B__C')
>>> for item in d:
...     print item.name
... 
a_object
b_object
c_object

>>> test = A.objects.select_related('B__C').__iter__().next()
>>> test.name
u'a_object'

, , , . , , .

Answer 2

, :

class A(models.Model):
    b = models.ForeignKey(B)
....
class B(models.Model):
....
class C(models.Model):
    b = models.OneToOneField(B)
    d = models.OneToOneField(D)
....
class D(models.Model):
....

,

A.objects.select_related('b__c__d').all()

A, , C . ,

A.objects.select_related('b__c').all()

Django ORM A, , C .

, select_related .