[Django] #33586: Cannot delete object with a foreign key dependency if the object also has a foreign key to a custom User model

[Django]

#33586: Cannot delete object with a foreign key dependency if the object also has a
foreign key to a custom User model
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Reporter: jaypoulz | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 4.0
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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I've created a reproducer app:
https://github.com/jaypoulz/rmigration_bug

After installing dependencies, run the repro.sh script:
https://github.com/jaypoulz/rmigration_bug/blob/main/repro.sh

All this script does is clear the db, run the initial migrations (which
will fail because no user exists yet), create a dummy super user, re-run
the migration so that it completes, and then run a reverse migration.

Object A has 2 ForeignKeys - the customer User and object B.
Object B is not important, but must exist in order to trigger the mismatch
in 'Fake' model comparison during the delete call.

https://github.com/jaypoulz/rmigration_bug/blob/main/rmigrate/migrations/0002_create_sample_objects.py#L15
{{{
ValueError: Cannot query "A object (1)": Must be "A" instance.
}}}

This appears to be because delete first checks if any B instances need to
be deleted (since it's a CASCADE on delete relationship).
Even though there are no instances of B, A cannot be deleted because the
model type returned by A via model lookup is not the same as the model
referenced by B.

For some reason, this only occurs when you have a custom User model.

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Ticket URL: <https://code.djangoproject.com/ticket/33586>
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