Avoid SET/DROP DEFAULT unless a field changes from null to non-null

[Matteo Pietro Russo]

If i consider to modify a column (ChaField) to add a default value as follow:

from django.db import models

class PersonModel(models.Model):
    nickname = models.CharField(default="noname", max_length=100, null=False)

makemigration insists on creating a new migration for this change. However, the migration on MYSQL shows:

BEGIN;
ALTER TABLE "core_personmodel" ALTER "nickname" SET DEFAULT 'noname';
ALTER TABLE "core_personmodel" ALTER "nickname" DROP DEFAULT;
COMMIT;

I think there is a mistake at line 735 in django/db/backends/base/schema.py

        # Drop the default if we need to
        # (Django usually does not use in-database defaults)
        if needs_database_default:
            sql = self.sql_alter_column % {
                "table": self.quote_name(model._meta.db_table),
                "changes": self.sql_alter_column_no_default % {
                    "column": self.quote_name(new_field.column),
                }
            }
            self.execute(sql)

If I "needs_database_default", why we need to drop default (sql_alter_column_no_default)? I read we use it to prevent extraneous DROP DEFAULT statements (an example is LONGTEXT in MySQL) because if database is able to use default we need to avoid to drop it.

[Tim Graham]

Django uses database defaults to set values on existing rows in a table. It doesn't leave the default values in the database so dropping the default is correct behavior. There might be an optimization not to set/drop the default in this case -- I'm not sure it's needed since the column isn't null. A separate ticket could be opened for this.

[Matteo Pietro Russo]

About default value, I read it in Django documentation:

"The default value is used when new model instances are created and a value isn’t provided for the field."

It's right if I use Django ORM, but I expect my field has rule I set in my model.

You said that "Django uses database defaults to set values on existing rows in a table", but if I migrate my table adding default value, migration doesn't change anything: It doesn't set default value on existing rows, it' doesn't change field (it is nullable yet and i haven't default).

from django.db import models

class PersonModel(models.Model):
    nickname = models.CharField(max_length=100, null=True)

If I change it:

    nickname = models.CharField(default="noname", max_length=100, null=False)

makemigrations builds a migration with this operation:

        migrations.AlterField(
            model_name='core_personmodel',
            name='nickname',
            field=models.CharField(default="noname", max_length=100, null=False),
        ),

If I execute migration:

Running migrations:
  Rendering model states... DONE
  Applying core.0004_migration... OK

but It's doesn't do anything.

[Tim Graham]

This is expected behavior. I should have said, "Django uses database defaults to set values on existing *null* rows in a table."

[Matteo Pietro Russo]

Ok, now I understand. For this reason, It's not a bug.

So I can't set default value on database via Django migrations, can I?

Default values are used by Django ORM, I think it's an useful improvement if we can apply it in db via migrations (as soon as possible). Do you agree?

MySQL backend uses base alter_field method so I can override it as proposal.

[Tim Graham]

No, it's not acceptable to overwrite existing data the default without the user's consent. You should use a RunPython operation to do that if needed.

[Matteo Pietro Russo]

I don't want to overwrite anything: i want to set default value in my db column (also if my table is empty).

ALTER TABLE "core_personmodel" ALTER "nickname" SET DEFAULT 'noname';

This statement doesn't modify data but column's specification, If I run it and then I run django migration, It drops my previous SQL: this is a mistake because I lost my defaults.

Why cannot I do that?

[Simon Charette]

This statement doesn't modify data but column's specification, If I run it and then I run django migration, It drops my previous SQL: this is a mistake because I lost my defaults.
Why cannot I do that?

Django's ORM don't use defaults defined at the database level and always specify the value when performing inserts instead.

There have been ​discussions to add support for database generated field values but nothing have been implemented so far.

[Matteo Pietro Russo]

Replying to Simon Charette:

This statement doesn't modify data but column's specification, If I run it and then I run django migration, It drops my previous SQL: this is a mistake because I lost my defaults.
Why cannot I do that?

Django's ORM don't use defaults defined at the database level and always specify the value when performing inserts instead.

There have been ​discussions to add support for database generated field values but nothing have been implemented so far.

This behaviour is correct but, I expect django migration doesn't delete my column specification (it DROPS my defaults value).

[Simon Charette]

Tim, I'll just repurpose this ticket for the optmization case.

​PR.

[Simon Charette]

This is duplicate of #27928.

[Marcin Nowak]

Django's ORM don't use defaults defined at the database level

But it should.

[Alessio Fachechi]

Replying to Marcin Nowak:

Django's ORM don't use defaults defined at the database level

But it should.

Totally agree. What's the reason for dropping them? I can understand the benefits of making Django handle its own default logic, but that doesn't mean dropping the database default, which can be used at last stage. Also the Django app is not always the only app accessing and writing to a database...

[Tim Graham]

Leaving database defaults would be backward incompatible and could result in surprising behavior. Field.default can be a callable which returns a non-static value that could be inappropriate as a long-running default.

#470 is a new feature ticket to allow using database defaults.