CircularDependencyError when squashing migrations

[no]

Project layout:

.
├── cheese
│   ├── init.py
│   ├── migrations
│   │   └── init.py
│   └── models.py (model Cheese, links to eggs.Store and spam.Employee)
├── djtest
│   ├── basemodel.py
│   ├── init.py
│   ├── settings.py
│   ├── urls.py
│   └── wsgi.py
├── eggs
│   ├── init.py
│   ├── migrations
│   │   └── init.py
│   └── models.py (model Store, links to spam.Employee)
├── manage.py
└── spam

├── init.py
├── migrations
│   └── init.py
└── models.py (model Employee, links to itself and eggs.Store; model Eggs, links to spam.Employee and cheese.Cheese)

Issue the following commands for the attached project:

$ python manage.py makemigrations
$ manage squashmigrations cheese 0003
$ manage squashmigrations eggs 0002

The following is raised:

django.db.migrations.graph.CircularDependencyError: [('spam', '0001_initial'), ('cheese', '0001_squashed_0003_cheese_updated_by'), ('spam', '0001_initial')]

[no]

Test project

[Simon Charette]

Thanks for the detailed report. Just to make sure, did you reproduce against 1.7-rc-2 or 1.7-rc-3?

[no]

It was 1.7-rc-3, The option wasn't available when I reported.

[Simon Charette]

Managed to reproduce.

[Andrew Godwin]

Ah, yes, squashing migrations does occasionally result in circular dependencies and there's no way we can solve this in 1.7 (it requires some specialised coding we just don't have). I'll add a note to the docs about this and bump it down from Release Blocker to a more general issue we need to solve next release.

[Andrew Godwin <andrew@…>]

In 459cfb523d8812081a623b29d0b8ab13bf87190a:

Update squashmigration docs to also mention circ dep errs (refs #23337)

[Andrew Godwin <andrew@…>]

In fbe45076234b1509a574a78a0f2b2dda602836db:

[1.7.x] Update squashmigration docs to also mention circ dep errs (refs #23337)

[bcail]

It looks like the first squashmigrations command for the cheese app results in a circular dependency, so a migrate or makemigrations command fails with the CircularDependencyError (even without squashing migrations in the eggs app).

Would it be helpful to add a check in the squashmigrations command to look at its output and throw an error instead of writing out a squashed migration that causes a CircularDependencyError?

[Sarah Boyce]

#35463 is a duplicate

[bcail]

​Here's an example PR that checks for circular dependencies when creating squashed migrations, and raises an error if there's a circular dependency. Is that a helpful direction? Would it be better to leave the squashed migration with the circular dependency, but print a warning for the user?

[Gordon Wrigley]

We have adopted a (rather complex) process to produce dual level squashes. The guts of the idea is instead of squashing each app to a single migration we squash to two migrations. The first one creates all the tables, the second one creates all the FK's. This resolves all circularity issues.

[bcail]

OK, do you want to open a thread on the forum, pitch that idea to the community, and see what people think?

[Gordon Wrigley]

Replying to bcail:

OK, do you want to open a thread on the forum, pitch that idea to the community, and see what people think?

We're going to run it again in a few weeks, maybe after that. I will feel more comfortable sharing the scripts we use if we've just recently used them.