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
37348	Removing `null=True` from GeneratedField should be a SQL no-op	Michal Porteš		"As of Django 6.1, having a GeneratedField declared with an explicit `null=True` causes the warning `(fields.W225) null has no effect on GeneratedField`.

Consider the following minimal example:

{{{
class NumericPrefix(models.Func):
    function = ""substring""
    template = r""%(function)s(%(expressions)s from '^\d+')""

class Foo(models.Model):
    code = models.CharField(max_length=50)
    code_num_prefix = models.GeneratedField(
        expression=NumericPrefix(""code""),
        output_field=models.CharField(max_length=50),
        db_persist=True,
        null=True,
    )
}}}

When I try to resolve the warning by removing `null=True`, the following happens:

* the `makemigrations` command prompts the dialog

{{{
It is impossible to change a nullable field 'code_num_prefix' on foo to non-nullable without providing a default. This is because the database needs something to populate existing rows.
Please select a fix:
 1) Provide a one-off default now (will be set on all existing rows with a null value for this column)
 2) Ignore for now. Existing rows that contain NULL values will have to be handled manually, for example with a RunPython or RunSQL operation.
 3) Quit and manually define a default value in models.py.
}}}

I select 2 because my table contains NULL values that I don’t want to change.

* `sqlmigrate` of the new migration outputs the following SQL:

{{{
BEGIN;
--
-- Alter field code_num_prefix on foo
--
ALTER TABLE ""foo_foo"" ALTER COLUMN ""code_num_prefix"" SET NOT NULL;
COMMIT;
}}}

* attempting to run `migrate` fails with

{{{
django.db.utils.IntegrityError: column ""code_num_prefix"" of relation ""foo_foo"" contains null values
}}}

(as mentioned above — my table contains NULL values)

These are all familiar steps when altering a normal column from nullable to non-nullable. But in this case it's unexpected and inconsistent with the warning's message.

[https://dryorm.xterm.info/generatedfield-vs-nulltrue This example] can be used to demonstrate that the DDL for the generated field is the same regardless of whether `null=True` is passed or not. Therefore I believe that this might be a bug and that removing `null=True` from GeneratedField should be a no-op at the database level.
"	Bug	new	Database layer (models, ORM)	6.1	Normal				Unreviewed	0	0	0	0	0	0