Implement RenameIndex in a backwards compatible way

[Markus Holtermann]

In order to eventually deprecate index_together we need a way to deal with old projects that have unnamed indexes. This proves to be a non-trivial problem. Andrew and I came up with these things to consider.

• RenameIndex(model, new_name, old_name=None, old_fields=None) where exactly one of old_name and old_field is given (old_name ^ old_fields)
• If the old_name is given we use RENAME INDEX if available
• Otherwise look at the state and drop existing indexes and create new the index with new name
• On MySQL (or other DBs) that don't support RENAME INDEX, provide SQL query to look up index name from information_schema by field names and pass in to DROP INDEX.
• If more than one index is found while identifying with field names, migrations must error out with an AmbiguityError
• If the autodetector finds an old, unnamed index and a new, named one matching field signature, issue a RenameIndex operation
• For backwards operations with unnamed old indexes, RenameIndex is a noop.

[Akshesh Doshi]

Is there a reason why we cannot just simply deprecate index_together.

My understanding is that the models file of the app will get modified from this

    class Meta:
        index_together = (('a', 'b'))

to this

    class Meta:
        indexes = [models.Index(fields=['a', 'b'])]

and everything should work fine (or not? Am I missing something?)

I need some help to see where RenameIndex comes into picture here.

[Markus Holtermann]

I just noticed that I actually never commented here. So, the reason is explained here: https://code.djangoproject.com/ticket/27236#comment:9

[Mariusz Felisiak]

On MySQL (or other DBs) that don't support RENAME INDEX, provide SQL query to look up index name from information_schema by field names and pass in to DROP INDEX.

RENAME INDEX is now supported on ​MySQL 5.7+ and ​MariaDB 10.5.2+.

[David Wobrock]

Hi there,

I tried to tackle this change in this ​PR.

Feel free to review it!
And maybe we'll be able to deprecate index_together in a next major release, as we wanted to do here https://code.djangoproject.com/ticket/27236 a few years back :)

[David Wobrock]

Integrated the review comments and split the PR into 2 parts:

1. Adding the RenameIndex operation ​https://github.com/django/django/pull/15677
2. Using the RenameIndex in the autodetector ​https://github.com/django/django/pull/15651

[David Wobrock]

Integrated review changes into the first PR ​implementing RenameIndex operation.

[Mariusz Felisiak <felisiak.mariusz@…>]

In eacd4977:

Refs #27064 -- Added RenameIndex migration operation.

[David Wobrock]

Rebased ​PR, ready for some review again :)
Even if it (most probably) won't be included to the coming 4.1 release.

[Mariusz Felisiak]

It's blocked by #33710.

[Mariusz Felisiak <felisiak.mariusz@…>]

In c6cec3c2:

Refs #27064 -- Made migrations generate RenameIndex operations when renaming Meta.indexes.

[Mariusz Felisiak <felisiak.mariusz@…>]

In a098cde9:

Refs #27064 -- Refactored out MigrationAutodetector.create_renamed_fields().

[Mariusz Felisiak <felisiak.mariusz@…>]

In 97f124f:

Refs #27064 -- Made migrations generate RenameIndex operations when moving indexes from index_together to Meta.indexes.