Create --update flag for makemigrations management command, mimicking South's one.

[Israel Saeta Pérez]

When I was developing with South I was able to use the --update flag for the schemamigration management command (​http://south.readthedocs.org/en/latest/commands.html#schemamigration) to refine the latest migration.

This was very convenient for iterative development. Could we have an equivalent of the --update flag for Django>=1.7 makemigrations?

(I've taken it from ​http://stackoverflow.com/questions/30487909/what-is-the-equivalent-of-souths-schemamigration-update-for-django-1-7)

[Tim Graham]

Andrew, could you offer your view.

[Andrew Godwin]

It's certainly possible, though more complicated than before in South because of the multi-app nature of makemigrations - but also easier because of the ease of programatically parsing and writing migration files.

What this would be, in essence, is something that runs makemigrations and glues the first migration it makes for each app to the most recent one that already exists. The pain points I forsee are:

• Migration files that have custom code (RunPython especially, but also any non-standard operation class) cannot be round-tripped through the parser and then out through the writer, so they could not be appended onto.

• Dependencies would have to be checked to make sure this merging of the new migrations into old does not cause dependency loops.

• It will occasionally have to just make new migrations anyway - it's more asking for a "best effort" attempt to make things work.

That said, I think it's not too complicated, and shares enough of these challenges with squashing (it's basically squashing the new migrations makemigrations creates into the last ones of the existing set), that we should aim to do it at some point.

[David Wobrock]

Hello,

I tried to tackle this ticket as I believe it would still be a pretty useful addition to makemigrations.
During development, I often see developers struggle with the manual steps to re-generate a migration while they are still iterating on their approach.

Here is the approach I've taken ​https://github.com/django/django/pull/15669
I tried to keep the implementation rather straightforward.

The main drawback I see is that it will add a new edge case when one updates a migration that has already been applied. This will create a bit of confusion, as it will become more tedious to undo the updated migration, since the reverse operations will try to undo things that weren't initially applied.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 3893fcdd:

Refs #24870 -- Refactored out get_relative_path() hook in makemigrations.

[Mariusz Felisiak <felisiak.mariusz@…>]

In e286ce17:

Fixed #24870 -- Added --update option to makemigrations command.