Migration re-add check constraint continuously when check condition contains a range object.

[Sigurd Ljødal]

A CheckConstraint with a Q(x__in=range(y, z)) condition is repeatedly deleted and re-added when running makemigrations.

models.CheckConstraint(
    check=models.Q(month__in=range(1, 13)),
    name='check_valid_month',
)

The generated migration looks like this, so I suspect that the issue is because the range is converted into a tuple:

    operations = [
        migrations.RemoveConstraint(
            model_name='monthlybudget',
            name='check_valid_month',
        ),
        migrations.AddConstraint(
            model_name='monthlybudget',
            constraint=models.CheckConstraint(check=models.Q(month__in=(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)), name='check_valid_month'),
        ),
    ]

A sample project with this issue can be found here:
​https://github.com/ljodal/djangocon-eu-2019

I'm open to fixing this issue myself, but I would like to clarify what would be a correct fix to this issue. I see at least two possible solutions, maybe three:

1. Keep the range method call in the generated migration file
2. Disallow using ranges in check constraints
3. (At least on PostgreSQL, we could use a range expression in the database too.)

[Mariusz Felisiak]

In check constraints you can use range lookup and it works fine, e.g.

models.CheckConstraint(
    check=models.Q(month__range=[1, 13]),
    name='check_valid_month',
)

also casting an iterator to a list works good, e.g.

models.CheckConstraint(
    check=models.Q(month__in=list(range(1, 13))),
    name='check_valid_month',
)

so in this case iterator is an issue. I would check later if this can be easily fix and if not we should document this limitation.

[Sigurd Ljødal]

Just to confirm, I modified the migration to use range() and makemigrations no longer detects any changes.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 2e38f20:

Fixed #30350 -- Prevented recreation of migration for operations with a range object.

Thanks to Mariusz Felisiak for helping with the patch.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 896cc719:

[2.2.x] Fixed #30350 -- Prevented recreation of migration for operations with a range object.

Thanks to Mariusz Felisiak for helping with the patch.

Backport of 2e38f2015aba224b68a91a3012b87223f3046bb6 from master.

[Mariusz Felisiak <felisiak.mariusz@…>]

In bc46e386:

Refs #30350 -- Doc'd support for range serialization in migrations.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 0098f26f:

[3.0.x] Refs #30350 -- Doc'd support for range serialization in migrations.

Backport of bc46e386c7aa496642d3ffc9e4f56ae5a46417a7 from master

[Mariusz Felisiak <felisiak.mariusz@…>]

In acc0d99e:

[2.2.x] Refs #30350 -- Doc'd support for range serialization in migrations.

Backport of bc46e386c7aa496642d3ffc9e4f56ae5a46417a7 from master