field automatically choosen as primary_key (primary_key=False do not work)

[Nicolas BREMOND]

I tried to use multi-table inheritance with a OneToOne field to the parent class in the child class but this field seems to be detected as primary_key by Django (there is only one row in the database).

The field has the same type as the parent link automatically created by Django and
the ​doc on inheritance tells that we can choose to use a such field for the inheritance relation with parent_link=True but I need the inheritance relation AND my other relation.
I tried to add parent_link=False but this don't change anything.
If I add null=True I got an error :

SystemCheckError: System check identified some issues:

ERRORS:
testapp.A.other: (fields.E007) Primary keys must not have null=True.
	HINT: Set null=False on the field, or remove primary_key=True argument.
from django.db import models

So I tried to add primary_key=False but that don't change anything.

There is a minimal code wich reproduce the error

models.py :

class Base(models.Model):
    pass

class A(Base):
    other=models.OneToOneField(Base,
            parent_link=False,#do not change anything
            primary_key=False,#do not change anything
            null=True,#raise a SystemCheckError
            related_name='other_base',
            )

I can continue my project if I replace the OneToOneField by a ForeignKey with unique=True but I have a warning with a hint to replace the ForeignKey by a OneToOneField :

System check identified some issues:

WARNINGS:
testapp.A.other: (fields.W342) Setting unique=True on a ForeignKey has the same effect as using a OneToOneField.
	HINT: ForeignKey(unique=True) is usually better served by a OneToOneField.

[Tim Graham]

Is this a duplicate of #26320?

[Simon Charette]

I'm fairly confident it is.

During the deprecation period you'll have to declare both OneToOneFields:

class A(Base):
    base_ptr = models.OneToOneField(Base, parent_link=True, primary_key=True)
    other = models.OneToOneField(Base, null=True, related_name='other_base')

Where the base_ptr field should be the named after the lower cased name of the parent-linked base followed by the '_ptr' suffix.

[Simon Charette]

@timgraham, It makes me wonder if we should adjust ​the deprecation warning raised on implicit non-parent_link OneToOneField promotion to mention either to add parent_link=True or how to declare the usual implicit parent_link as the actions to take in order to have both fields available is non-trivial from the deprecation message.

For example, in the reported case the message would be:

Implicit parent link is deprecated, either add parent_link=True to A.other or declare base_ptr = models.OneToOneField(Base, parent_link=True, primary_key=True) before it during the deprecation period

[Tim Graham]

Seems like it wouldn't hurt.

[Nicolas BREMOND]

Yes, this is a duplicate of #26320, sorry…
Thanks for the help !

[Nicolas BREMOND]

I don't know if I have to reply here, on #26320 or on a new ticket, but the solution given by charette on comment:2 does not really work.

The problem on fields is solved but I think there is still a bug during objects instanciation.

#models.py
from django.db import models

class Base(models.Model):
    def __str__(self):#indicate if the object is a member of a subclass
        try:
            a=self.a
            return "a A pk="+str(self.pk)+" (other="+str(a.other.pk)+")"
        except A.DoesNotExist:
            pass
        try:
            z=self.z
            return "not a A but a Z pk="+str(self.pk)
        except Z.DoesNotExist:
            pass
        return "not a A neither a Z pk="+str(self.pk)

class A(Base):
    base_ptr = models.OneToOneField(Base, parent_link=True, primary_key=True)
    other = models.OneToOneField(Base, related_name='other_base') #I do not have null=True

class Z(Base):
    pass

I can manipulate the Z objects :
adding a new Z :

>>> z=Z()
>>> z.save()
>>> Base.objects.all()
[<Base: not a A but a Z pk=1>]

if I try to do the same thing with a A (with a new database):

>>> other=Base()
>>> other.save()
>>> a=A(other=other)
>>> a.save()
>>> a
<A: not a A neither a Z pk=None> #a is supposed to have a pk which is not None 
>>> a.other
<Base: a A pk=1 (other=1)>#other is supposed to be a base and not a A
>>> Base.objects.all()
[<Base: a A pk=1 (other=1)>]#I was expecting two objects

It seems to be possible to change a Base to a A:

>>> other=Base()
>>> other.save()
>>> b=Base()
>>> b.save()
>>> Base.objects.all()
[<Base: not a A neither a Z pk=1>, <Base: not a A neither a Z pk=2>]
>>> a=A(other=other,base_ptr=b)
>>> a.save()
>>> a
<A: not a A neither a Z pk=2>#str(a) should return "a A" but this work if we reload the object
>>> a.other
<Base: not a A neither a Z pk=1>
>>> Base.objects.all()
<Base: not a A neither a Z pk=1>, <Base: a A pk=2 (other=1)>]

This is maybe also deprecated but it can be a bug in the next version and I didn't see a fix in ​PR (I quickly look the pull request, maybe I'm wrong)

[Tim Graham]

Yes, a new ticket is more appropriate. #9982 may be related.