Model o2o inheritance with abstract models does not "evaluate" a lazy relationship

[BeryCZ]

When I define models like this

class AbstractPage(models.Model):
    mtime = models.DateTimeField(
        verbose_name="Last modification at",
        auto_now=True,
    )

    class Meta:
        abstract = True


class AbstractPageType(models.Model):
    page = models.OneToOneField(
        "Page",
        verbose_name="ID",
        primary_key=True,
        on_delete=models.CASCADE,
        related_name="%(class)s",
        parent_link=True,
    )
    title = models.CharField(
        verbose_name="Title",
        max_length=255,
        null=False,
        blank=False,
    )

    class Meta:
        abstract = True


class Page(AbstractPage):
    pass


class ArticlePage(AbstractPageType, Page):
    pass

And then I do

ArticlePage.objects.order_by("-mtime").all()

I get

  File "django/db/models/manager.py", line 87, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "django/db/models/query.py", line 1701, in order_by
    obj.query.add_ordering(*field_names)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "django/db/models/sql/query.py", line 2249, in add_ordering
    self.names_to_path(item.split(LOOKUP_SEP), self.model._meta)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "django/db/models/sql/query.py", line 1777, in names_to_path
    path_to_parent = opts.get_path_to_parent(model)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "django/db/models/options.py", line 755, in get_path_to_parent
    targets = (final_field.remote_field.get_related_field(),)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "django/db/models/fields/reverse_related.py", line 311, in get_related_field
    field = self.model._meta.get_field(self.field_name)
            ^^^^^^^^^^^^^^^^

Exception Type: AttributeError
Exception Value: 'str' object has no attribute '_meta'

It works when I do

class ArticlePage(AbstractPageType, Page):
    page = models.OneToOneField(
        "Page",
        verbose_name="ID",
        primary_key=True,
        on_delete=models.CASCADE,
        related_name="%(class)s",
        parent_link=True,
    )

but I would prefer not to have to define page field in every PageType.

[Natalia Bidart]

Hello BeryCZ, thank you for this report. I've analyzed your models and I fear that the issue is the complex inheritance that you have proposed. Specifically, you cannot have a model (ArticlePage) that both inherits from Page and has a OneToOneField to Page. Without understanding your use case, it feels like a possible solution would be for your "pages" to have a FK to "page types" (instead of the other way around with the 1-1 field).

Your model definitions create an inheritance conflict:

1. AbstractPageType expects a separate Page instance (this means every model that inherits from AbstractPageType must have a separate Page instance linked via the page field).
2. ArticlePage is also inheriting Page, which means Django expects ArticlePage itself to be a Page instance.

Django sees two competing ways to resolve ArticlePage:

• ArticlePage is a Page (due to model inheritance), or
• ArticlePage has a separate Page instance (due to the page OneToOneField).

The error is caused because Django tries to retrieve the mtime field, which exists on Page (inherited via AbstractPage). But because ArticlePage also has a OneToOneField called page, Django gets confused about how to resolve mtime. Instead of treating page as a relationship to another Page instance, Django interprets it as a string.

Solutions are either to remove the direct inheritance to Page or remove the page field from AbstractPageType.

If you have further questions, there are several user support channels available. Please refer to TicketClosingReasons/UseSupportChannels for ways to get help.

[BeryCZ]

Hello Natalia, thanks for your response, but I think you either misunderstood or I'm missing something that's not clear from your answer.

Based on the docs ​https://docs.djangoproject.com/en/5.1/topics/db/models/#multi-table-inheritance I can override the primary key on ArticlePage so instead of page_ptr I can have just page. So that's what I'm doing, it works when I put it in ArticlePage model, but not when I put it the AbstractPageType.
I've just asked on discord and klove said it should be ok to name the pk page.
So I even tried to remove my custom pk field and checked ArticlePage.page and article_page_object.page and both raise AttributeError, so by default page is not there.

My use case is - I want to have the "parent" model Page from which I can MTI "submodels" like Article, Category, Product, Reference, whatever. The Page model will then be used for references in other models (galeries, urls, tree-maps, ...).

So, am I missing something, or is this a bug? I'm not saying it's an easy-to-fix bug :) but I honestly think it is a bug.
I'm not even sure where the lazy load of to relationship is, or why it doesn't work in abstract models, otherwise I would try to fix it and PR.

Thank you,
Bery

[Simon Charette]

I believe your AbstractPageType definition is invalid as it should not allowed to define a parent_link on a model that doesn't directly inherit from the referenced model even when abstract.

The correct way to do this is to have the abstract model defining the parent link subclass the concrete model (abstract MTI). This means you should be able to achieve what you're after by re-organizing your model definition to make sure AbstractPageType does inherit from Page

class AbstractPage(models.Model):
    mtime = models.DateTimeField(
        verbose_name="Last modification at",
        auto_now=True,
    )

    class Meta:
        abstract = True

class Page(AbstractPage):
    pass

class AbstractPageType(Page):
    page = models.OneToOneField(
        "Page",
        verbose_name="ID",
        primary_key=True,
        on_delete=models.CASCADE,
        related_name="%(class)s",
        parent_link=True,
    )

    class Meta:
        abstract = True

class ArticlePage(AbstractPageType):
    pass

class OtherPage(AbstractPageType):
    pass

This is not something Django could be easily adjusted to warn you about as no registry of abstract models is kept and thus system checks cannot be run against them.

[Simon Charette]

So I tried adapting BaseModel.__new__ to confirm this was effectively never supported with

django/db/models/base.py

@@ -243 +243 @@
             for field in base._meta.local_fields:
                 if isinstance(field, OneToOneField) and field.remote_field.parent_link:
                     related = resolve_relation(new_class, field.remote_field.model)
-                    parent_links[make_model_tuple(related)] = field
+                    related_model_tuple = make_model_tuple(related)
+                    if abstract and not any(
+                        make_model_tuple(parent) == related_model_tuple
+                        for parent in parents
+                        if hasattr(parent, "_meta")
+                    ):
+                        raise TypeError(
+                            "Abstract models must directly inherit from parent "
+                            "links they refer to."
+                        )
+                    parent_links[related_model_tuple] = field
 
         # Track fields inherited from base models.
         inherited_attributes = set()

And the suited failed on ​these models which git-blame led me to #20883 (163a34ce4bc1086b346a52c7271f48d2c207f710) which makes me believe this either never fully worked (notice the tests are very minimal) or that the latest changes to abstract model inheritance field attribution in #24305 (85ef98dc6ec565b1add417bd76808664e7318026).

Given we have models in the suite I think we should at least bisect to determine if this was ever properly supported but so minimally tested that #24305 silently broke it. If it never truly worked then I'd suggest we add this TypeError otherwise if it was broken by #24305 we should consider it like a long standing regression.

[BeryCZ]

Hello Simon, thank you for your response.
The re-organization of the models isnt really a solution, because in my case the abstracts are in a reusable app and then models are project specific. I dont like to have concrete models in a reusable app and never be able to rewrite them, but I should have the AbstractPageType in the reusable app already.

I would really like if it was possible to define the parent_link's field name in the abstract, so I don't have to define it on all PageType models, because page is much more comfortable to work with than page_ptr.