Removing Meta.db_table generates migration with wrong rename in comment (None)

[Daniel]

# myapp.models.py

#Old model
class MyModel(models.Model):
    ...
    class Meta:
        db_table = 'legacy_table'

#New model
class MyModel(models.Model):
    ...

# makemigrations output:
- Rename table for mymodel to None

# sqlmigrate output:
BEGIN;
--
-- Rename table for mymodel to None
--
RENAME TABLE `legacy_table` TO `myapp_mymodel`;

COMMIT;

[Tim Graham]

I think we ​don't have the needed information to put the actual name in the comment, but perhaps we could replace None with something like (default).

[PREMANAND]

I'm thinking the following, please review and provide your comments.

When someone provides "None" as the new table name, we won't detect that as a change and just ignore it.

​https://github.com/django/django/blob/fb4272f0e6bbdaa3e19ed5fde59fdb5ab5a33baf/django/db/migrations/autodetector.py#L962

This is the new change that works.

if old_db_table_name != new_db_table_name and new_db_table_name != None:
                self.add_operation(
                    app_label,
                    operations.AlterModelTable(
                        name=model_name,
                        table=new_db_table_name,
                    )
                )

We don't have the same problem in the initial creation of the table if the table name is "None". It defaults to the app name as normal.

[Tim Graham]

The change I proposed in comment 1 is about modifying AlterModelTable.describe(). The autodetector shouldn't be changed like that because then a migration changing the table name back to the default wouldn't be created.

[PREMANAND]

​PR

[Tim Graham <timograham@…>]

In 5da7e3f:

Fixed #26114 -- Fixed AlterModelTable.describe() if db_table is None.