Migration re-add constraints when check condition contains a dict_keys object.

[bcail]

A CheckConstraint using dict_keys like this has an issue:

models.CheckConstraint(
        name='animal_values',
        check=models.Q(animal__in={'c': 'cat', 'd': 'dog'}.keys())
)

It continually creates migrations and prompts you to create another migration - it doesn't realize that the migration has already been generated and applied.

If I change it to a list like this, it works - the problem goes away:

models.CheckConstraint(
        name='animal_values',
        check=models.Q(animal__in=list({'c': 'cat', 'd': 'dog'}.keys()))
)

Should Django properly handle dict_keys, or should the docs say that they have to be converted to a list?

[David Sanders]

Without offering opinion on whether this is a bug or not, here are some notes:

• Migration change detection compares the loaded state from migration files with the current project's state
• Migrations are serialised before written to disk
• When the project's state is loaded it's not serialised first
• tuples & lists are serialised independently; all other iterables get serialised into tuples (see django/db/migrations/serializer.py)

So we have a situation where migrations is comparing serialized state to pre-serialized state.

While this could potentially be a bug, the workaround is to simply use a list and "fixing" this issue could break expected current behaviour? 🤔

[Mariusz Felisiak]

I think it's worth fixing (check out a similar ticket #30350).

[bcail]

Thanks David and Mariusz.

I added print statements to MigrationAutodetector.create_altered_constraints, and that confirms the serialized vs non-serialized states:

old_constraints=[<CheckConstraint: check=(AND: ('animal__in', ('c', 'd'))) name='animal_values'>]
new_constraints=[<CheckConstraint: check=(AND: ('animal__in', dict_keys(['c', 'd']))) name='animal_values'>]

I'm not sure how to best fix this, though.

[David Sanders]

I have a draft PR which serialises during the change detection: ​https://github.com/django/django/pull/17114

I'm not particularly fond of this though. For one thing this probably would've masked the real issue behind #30350? 🤔 It also only narrowly focuses on one part of the project state.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 76c3e310:

Fixed #34744 -- Prevented recreation of migration for constraints with a dict_keys.

Co-authored-by: Mariusz Felisiak <felisiak.mariusz@…>