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
34366	Migration optimizer does not reduce multiple AlterField	Laurent Tramoy	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]
}}}
"	Cleanup/optimization	closed	Migrations	dev	Normal	fixed			Ready for checkin	1	0	0	0	0	0