id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
9982	Inconsistent behavior on model save depending on whether OneToOneField is a primary key	sean@…		"Hi,
When using a OneToOneField to make one model extend another, I noticed a behavioral inconsistency depending on whether the related model's OneToOneField was also the primary key or not.

The inconsistency is to do with when save() can be called on model instances.
Consider the following example models:

{{{
class BaseModel(models.Model):
    pass

class ExtendedModel(models.Model):
    link = models.OneToOneField(BaseModel)
}}}

It is not possible to first instantiate these models and later on save them - an IntegrityError would be raised when saving the ExtendedModel. E.g., 
{{{
o1 = BaseModel()
o2 = ExtendedModel(link=o1)
o1.save()
o2.save()
}}}
when o2 is saved it will raise an IntegrityError because it doesn't have a value for the primary key of the model it is related to. I think o1's primary key is copied when o2 is initialized, and is None at this point because o1 isn't saved yet.

However, if the definition of ExtendedModel is changed so that the OneToOneField is also the primary key, it now becomes possible to save the model instances in this order. To test this I changed the model definition as follows:
{{{
class ExtendedModel(models.Model):
    link = models.OneToOneField(BaseModel, primary_key=True)
}}}

When using this model definition it seems that o1's primary key is copied when o2 is saved, as opposed to when o2 is initialized. This inconsistency caused a lot of head scratching here.

Thanks!
Sean


"	Bug	closed	Database layer (models, ORM)	dev	Normal	fixed	OneToOneField primary_key IntegrityError	Can Sarıgöl	Accepted	0	0	0	0	0	0