makemigrations does not detect custom field subclassing change

[Phil Krylov]

Example:

class MyLongTextField(models.CharField):
    def __init__(self, *args, **kwargs):
        kwargs["max_length"] = 2000
        super(MyLongTextField, self).__init__(*args, **kwargs)

After changing this to

class MyLongTextField(models.TextField):
    def __init__(self, *args, **kwargs):
        kwargs["max_length"] = 2000
        super(MyLongTextField, self).__init__(*args, **kwargs)

manage.py makemigrations detects no changes.

[Tim Graham]

I'm not sure how to handle this.

[Phil Krylov]

I think that expected behaviour is to generate an ALTER TABLE table_name ALTER COLUMN column_name TYPE TEXT migration operation. However, I'm not familiar enough with migration internals to predict what it takes to detect a change like this.

[Baylee Feore]

After discussion with Markus at PyCon sprints 2016:

This isn't possible. The closest we get is ​here, where we could add a check for get_internal_type() of the two fields. But that won't work because the old state and new state are dynamically generated from the field definition in the migration files. The only way to handle this would be if we saved internal type as part of the migrations.

Propose closing this in favor of adding documentation in #26702

[Tim Graham]

For future reference, it would probably be fine to reclassify this ticket as a documentation issue, but since you've already created another issue, I'll close this.