Migration optimizer does not reduce multiple AlterField

[Laurent Tramoy]

Let's consider the following operations:

operations = [
    migrations.AddField(
        model_name="book",
        name="title",
        field=models.CharField(max_length=256, null=True),
    ),
    migrations.AlterField(
        model_name="book",
        name="title",
        field=models.CharField(max_length=128, null=True),
    ),
    migrations.AlterField(
        model_name="book",
        name="title",
        field=models.CharField(max_length=128, null=True, help_text="help"),
    ),
    migrations.AlterField(
        model_name="book",
        name="title",
        field=models.CharField(max_length=128, null=True, help_text="help", default=None),
    ),
]

If I run the optimizer, I get only the AddField, as we could expect. However, if the AddField model is separated from the AlterField (e.g. because of a non-elidable migration, or inside a non-squashed migration), none of the AlterField are reduced:

optimizer.optimize(operations[1:], "books") 

[<AlterField  model_name='book', name='title', field=<django.db.models.fields.CharField>>,
 <AlterField  model_name='book', name='title', field=<django.db.models.fields.CharField>>,
 <AlterField  model_name='book', name='title', field=<django.db.models.fields.CharField>>]

Indeed, the AlterField.reduce does not consider the the case where operation is also an AlterField.
Is this behaviour intended? If so, could it be documented?
Otherwise, would it make sense to add something like

        if isinstance(operation, AlterField) and self.is_same_field_operation(
            operation
        ):
            return [operation]

[Simon Charette]

Your analysis is correct Laurent, the reduction of multiple AlterField against the same model is simply not implemented today hence why you're running into this behaviour.

Given you're already half way there ​I would encourage you to submit a PR that adds these changes and ​an optimizer regression test to cover them if you'd like to see this issue fixed in future versions of Django.

[Laurent Tramoy]

Thanks Simon, I submitted a PR.

[Bhuvnesh]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In 2276ec8c:

Fixed #34366 -- Reduced AlterField operations when optimizing migrations.