Inline forms break when the foreign model does not inherit directly from models.Model

[mkibbel@…]

This sample code example demonstrates the problem:

from django.db import models
from django.contrib import admin

class Base(models.Model):
   base_name = models.CharField(
      "Base Name",
      max_length = 40,
   )

class Child(Base):
   child_name = models.CharField(
      "Child Name",
      max_length = 40,
   )

class Inline(models.Model):
   child = models.ForeignKey(
      Child,
   )
   inline_name = models.CharField(
      "Inline Name",
      max_length = 40,
   )

class InlineAdmin(admin.StackedInline):
   model = Inline

class ChildAdmin(admin.ModelAdmin):
   inlines = (InlineAdmin,)

try:
   admin.site.register(Child, ChildAdmin)
except admin.sites.AlreadyRegistered:
   pass

When the admin's add_view for the "Child" class runs, a DoesNotExist exception is raised. If any of the following changes is made, the problem does not present itself:

• removing the "edit_inline" functionality by removing "inlines" from ChildAdmin (not an acceptable workaround)
• making the Child model inherit from models.Model (better, but still very inconvenient)

I traced the problem back to the get_query_set function in django.newforms.models.BaseInlineFormset, which reads as such:

    def get_queryset(self):
        """
        Returns this FormSet's queryset, but restricted to children of 
        self.instance
        """
        kwargs = {self.fk.name: self.instance}
        return self.model._default_manager.filter(**kwargs)

Here, self.instance == Child(), an unsaved Child model instance (with no primary key), so the resulting call to filter is looking up an Inline object by an undefined primary key on the Child object. I have no idea why this problem does not present itself when Child inherits from models.Model, but I do have a simple patch to fix this bug.

[mkibbel@…]

Patch to BaseInlineFormset.get_query_set

[Brian Rosner]

Let me do a quick brain dump here. Don't want to forget what I have tracked down thus far.

• The DoesNotExist is coming from source:/branches/newforms-admin/django/db/models/fields/related.py#L231
• The above is being called due to source:/branches/newforms-admin/django/db/models/fields/related.py#L137
• Ultimately, Choice().poll does the right thing and raises the DoesNotExist exception. However when translating this into a queryset Choice.objects.filter(poll=Poll()) when Poll does not subclass something will evaluate to an empty list. But when the parent class (multi-table inheritance ) is introduced it will throw this DoesNotExist exception.
• The patch here does indeed fix the problem, but I begin to wonder if this should be fixed upstream?

[Marc Fargas]

As per IRC conversation it seems this should be fixed up in the stack, so changing to trunk and querysets component.

[Marc Garcia]

So, if it's not a newforms-admin merge blocker bug, milestone should be 1.0 instead of 1.0 alpha.

[Brian Rosner]

failing tests

[Malcolm Tredinnick]

(In [8061]) Allow for matches against unsaved objects in querysets (which will therefore
match nothing). This allows for some more straightforward code in the admin
interface.

Fixed #7488 (all the debugging there was done by Brian Rosner, who narrowed it
down to the item in this patch).

[Jacob]

Milestone 1.0 deleted

[Mariusz Felisiak <felisiak.mariusz@…>]

In 18245b94:

Refs #7488, Refs #19524 -- Removed obsolete ModelInheritanceTest.test_issue_7488() test.

Obsolete since e9c24bef74e55729b190cf07e0ac452aa4c86fcd.