only() broken for proxy models

[michal.modzelewski@…]

Usage of deferred fields with proxy models is broken.

For example when using the following models

class Item(models.Model):
    name = models.CharField(max_length=15)
    text = models.TextField(default="xyzzy")
    value = models.IntegerField()
    other_value = models.IntegerField(default=0)

class Proxy(Item):
    class Meta:
        proxy = True

we create an item with

>>> proxy = Proxy.objects.create(name="proxy", value=42)

Now depending on whether we call only() with 'id' or not we get different behaviour.
Without 'id' we get an endless loop.

>>> deferredproxy = Proxy.objects.only('value').get(pk=proxy.pk)
>>> deferredproxy.name
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    deferredproxy.name
  File "/home/michal/.local/lib/python2.7/site-packages/django/db/models/query_utils.py", line 100, in __get__
    cls._base_manager.filter(pk=instance.pk).only(name).using(
  File "/home/michal/.local/lib/python2.7/site-packages/django/db/models/base.py", line 426, in _get_pk_val
    return getattr(self, meta.pk.attname)
  File "/home/michal/.local/lib/python2.7/site-packages/django/db/models/query_utils.py", line 100, in __get__
    cls._base_manager.filter(pk=instance.pk).only(name).using(
...
  File "/home/michal/.local/lib/python2.7/site-packages/django/db/models/base.py", line 426, in _get_pk_val
    return getattr(self, meta.pk.attname)
RuntimeError: maximum recursion depth exceeded

With 'id' we get the wrong value when accessing a deferred field.

>>> deferredproxy = Proxy.objects.only('id', 'value').get(pk=proxy.pk)
>>> deferredproxy.name
3

This value happens to be the object id.

>>> proxy.id
3

Comparing the objects returned with examples using the base model.

>>> Proxy.objects.only('other_value').get(pk=proxy.pk).__dict__
{'_state': <django.db.models.base.ModelState object at 0x89bd58c>, 'other_value': 3}
>>> Item.objects.only('other_value').get(pk=proxy.pk).__dict__
{'_state': <django.db.models.base.ModelState object at 0x89bd80c>, 'id': 3, 'other_value': 0}
>>> Proxy.objects.only('name', 'text', 'value', 'other_value').get(pk=proxy.pk).__dict__
{'text': u'proxy', '_state': <django.db.models.base.ModelState object at 0x89bda8c>, 'name': 3, 'value': u'xyzzy', 'other_value': 42}
>>> Item.objects.only('name', 'text', 'value', 'other_value').get(pk=proxy.pk).__dict__
{'name': u'proxy', 'text': u'xyzzy', '_state': <django.db.models.base.ModelState object at 0x89bdcec>, 'value': 42, 'other_value': 0, 'id': 3}

'id' is missing from the __dict__ for the proxy models, and all the values are shifted.

[michal.modzelewski@…]

Patch for this bug, with regression tests

[Luke Plant]

I don't think this is a release blocker, as it is not a regression, or a serious bug with a new feature in 1.3 that would have blocked it.

[Michał Modzelewski <michal.modzelewski@…>]

Yes, i originally thought that this could result in an unintentional loss of data if you saved such a broken model instance, but since the pk value is missing from the __dict__ saving will just create a new record in the database.

[Michał Modzelewski]

Updated patch for trunk

[kgibula]

Patch applies cleanly and the described issue is fixed.

[Ramiro Morales]

In [16228]:

Fixed #15790 -- Fixed QuerySet only() and defer() methods behavior with proxy models. Thanks Michal Modzelewzki for the report and patch.

[Jacob]

Milestone 1.4 deleted