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
30032	Allow expressions to be used for default	Gavin Wahl	Johannes Maron	"I have this field:
{{{
    foo = models.DateTimeField(default=timezone.now)
}}}
I would like to modify it to use TransactionNow instead:
{{{
    foo = models.DateTimeField(default=TransactionNow())
}}}
This migration operation is generated:
{{{
        migrations.AlterField(
            model_name='somemodel',
            name='foo',
            field=models.DateTimeField(default=django.contrib.postgres.functions.TransactionNow()),
        ),
}}}

When run, this error is thrown:

{{{
  File ""./manage.py"", line 29, in <module>
    execute_from_command_line(sys.argv)
  File ""django/core/management/__init__.py"", line 381, in execute_from_command_line
    utility.execute()
  File ""django/core/management/__init__.py"", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File ""django/core/management/base.py"", line 316, in run_from_argv
    self.execute(*args, **cmd_options)
  File ""django/core/management/base.py"", line 353, in execute
    output = self.handle(*args, **options)
  File ""django/core/management/base.py"", line 83, in wrapped
    res = handle_func(*args, **kwargs)
  File ""django/core/management/commands/migrate.py"", line 203, in handle
    fake_initial=fake_initial,
  File ""django/db/migrations/executor.py"", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File ""django/db/migrations/executor.py"", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File ""django/db/migrations/executor.py"", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File ""django/db/migrations/migration.py"", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File ""django/db/migrations/operations/fields.py"", line 216, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File ""django/db/backends/base/schema.py"", line 523, in alter_field
    old_db_params, new_db_params, strict)
  File ""django/db/backends/postgresql/schema.py"", line 122, in _alter_field
    new_db_params, strict,
  File ""django/db/backends/base/schema.py"", line 626, in _alter_field
    old_default = self.effective_default(old_field)
  File ""django/db/backends/base/schema.py"", line 239, in effective_default
    return field.get_db_prep_save(default, self.connection)
  File ""django/db/models/fields/__init__.py"", line 790, in get_db_prep_save
    return self.get_db_prep_value(value, connection=connection, prepared=False)
  File ""django/db/models/fields/__init__.py"", line 1429, in get_db_prep_value
    value = self.get_prep_value(value)
  File ""django/db/models/fields/__init__.py"", line 1408, in get_prep_value
    value = super().get_prep_value(value)
  File ""django/db/models/fields/__init__.py"", line 1268, in get_prep_value
    return self.to_python(value)
  File ""django/db/models/fields/__init__.py"", line 1369, in to_python
    parsed = parse_datetime(value)
  File ""django/utils/dateparse.py"", line 106, in parse_datetime
    match = datetime_re.match(value)
TypeError: expected string or bytes-like object
}}}

If I create the field foo from scratch, with an AddField operation instead of AlterField, it works as expected.

I should be able to modify a field to use TransactionNow() as a default."	New feature	assigned	Database layer (models, ORM)	dev	Normal			Johannes Maron Brian Helba Ülgen Sarıkavak Sage Abdullah	Accepted	1	0	0	1	0	0