id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
33586	Applying RunPython backward results in ValueError when involved models are referenced through multiple foreign keys	Jeremy Poulin	Sarah Boyce	"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 superuser, re-run the migration so that it completes, and then run a reverse migration.

Object A has a single foreign key to a custom User object.
Object B has a foreign key (many to one) relationships to object A. This object never needs to be instantiated, and in fact, only exists to trigger a 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 object A is referenced by any models that have a CASCADE on delete relationship, and discovers object B. It then compares the model identifier of object B's reference to object A with the instance type of the object that is going to be deleted (also object A).

For some reason, these identifiers do not match.

In other words, 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 type referenced by B.

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

If you comment out the User model, the issue no longer occurs. I've created a branch to prove this as well:
https://github.com/jaypoulz/rmigration_bug/compare/no-custom-user
"	Bug	assigned	Migrations	4.0	Normal			Simon Charette Ülgen Sarıkavak Enrico Zini mag123c	Accepted	1	0	1	1	0	0