KeyError when adding new objects two relations deep via inlines but the intermediate model is not registered with the admin

[Jacob Walls]

With these models:

class Analysis(models.Model): ...

class Song(models.Model):
    users = models.ManyToManyField("auth.User")
    analysis = models.ForeignKey("Analysis", on_delete=models.CASCADE, null=True)

And these admins:

class SongInline(admin.TabularInline):
    model = Song


class AnalysisAdmin(admin.ModelAdmin):
    inlines = [SongInline]

admin.site.register([Analysis], AnalysisAdmin)

1. Admin: Analysis: Add
2. Click the "+" in the Users column of the Songs inline
3. Fill out the field
4. Notice Song is in the URL as the source_model arg. This will be a problem, since Song is not registered with the admin.
5. Submit

  File "/Users/jacobwalls/django/django/contrib/admin/options.py", line 711, in wrapper
    return self.admin_site.admin_view(view)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/utils/decorators.py", line 191, in _view_wrapper
    result = _process_exception(request, e)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/utils/decorators.py", line 189, in _view_wrapper
    response = view_func(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/views/decorators/cache.py", line 79, in _view_wrapper
    response = view_func(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/contrib/admin/sites.py", line 247, in inner
    return view(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/utils/decorators.py", line 47, in _wrapper
    return bound_method(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/views/decorators/debug.py", line 142, in sensitive_post_parameters_wrapper
    return view(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/utils/decorators.py", line 191, in _view_wrapper
    result = _process_exception(request, e)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/utils/decorators.py", line 189, in _view_wrapper
    response = view_func(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/contrib/auth/admin.py", line 119, in add_view
    return self._add_view(request, form_url, extra_context)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/contrib/auth/admin.py", line 147, in _add_view
    return super().add_view(request, form_url, extra_context)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/contrib/admin/options.py", line 1984, in add_view
    return self.changeform_view(request, None, form_url, extra_context)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/utils/decorators.py", line 47, in _wrapper
    return bound_method(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/utils/decorators.py", line 191, in _view_wrapper
    result = _process_exception(request, e)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/utils/decorators.py", line 189, in _view_wrapper
    response = view_func(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/contrib/admin/options.py", line 1842, in changeform_view
    return self._changeform_view(request, object_id, form_url, extra_context)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/contrib/admin/options.py", line 1900, in _changeform_view
    return self.response_add(request, new_object)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/contrib/auth/admin.py", line 251, in response_add
    return super().response_add(request, obj, post_url_continue)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jacobwalls/django/django/contrib/admin/options.py", line 1431, in response_add
    source_admin = self.admin_site._registry[source_model]
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Exception Type: KeyError at /admin/auth/user/add/
Exception Value: <class 'earTrain.models.Song'>

Regression in b1ffa9a9d78b0c2c5ad6ed5a1d84e380d5cfd010.

[Sean Helvey]

Eek! I'm not familiar with the triage process but believe this patch would add an if statement to check for that. It worked for me locally:

-                    source_admin = self.admin_site._registry[source_model]
-                    form = source_admin.get_form(request)()
-                    if self.opts.verbose_name_plural in form.fields:
-                        field = form.fields[self.opts.verbose_name_plural]
-                        for option_value, option_label in field.choices:
-                            # Check if this is an optgroup (label is a sequence
-                            # of choices rather than a single string value).
-                            if isinstance(option_label, (list, tuple)):
-                                # It's an optgroup:
-                                # (group_name, [(value, label), ...])
-                                optgroup_label = option_value
-                                for choice_value, choice_display in option_label:
-                                    if choice_display == str(obj):
-                                        popup_response["optgroup"] = str(optgroup_label)
-                                        break
+                    if source_model in self.admin_site._registry:
+                        source_admin = self.admin_site._registry[source_model]
+                        form = source_admin.get_form(request)()
+                        if self.opts.verbose_name_plural in form.fields:
+                            field = form.fields[self.opts.verbose_name_plural]
+                            for option_value, option_label in field.choices:
+                                # Check if this is an optgroup (label is a
+                                # sequence of choices rather than a single
+                                # string value).
+                                if isinstance(option_label, (list, tuple)):
+                                    # It's an optgroup:
+                                    # (group_name, [(value, label), ...])
+                                    optgroup_label = option_value
+                                    for choice_value, choice_display in option_label:
+                                        if choice_display == str(obj):
+                                            popup_response["optgroup"] = str(
+                                                optgroup_label
+                                            )
+                                            break

How can I help?

[Natalia Bidart]

Replying to Sean Helvey:

How can I help?

If you have reproduced the reported issue, you can accept the ticket saying so. If you also want to work on the ticket, assign it to yourself after accepting. More details on the triage process here: ​https://docs.djangoproject.com/en/dev/internals/contributing/triaging-tickets/

Thank you!

[Sean Helvey]

Created PR: ​https://github.com/django/django/pull/20679.