MigrationWriter does not support writing run_before

[Mikuláš Poul]

The MigrationWriter class implements writing a instance of django.db.migrations.migration.Migration. It supports serializing all the options it could have except for run_before. I recently made a PR for a third-party package on remaking migrations (a form of squashing), and had to subclass the writer to be able to include run_before. I think MigrationWriter should support all options available on Migration, to make working with migrations easier.

[Simon Charette]

I'm not sure I would qualify it as a bug given the framework itself doesn't make use of this option internally when creating migrations so it had little use in adding support for it.

It's a small non-invasive feature request though, as patch demonstrates, so I don't think it warrants a larger discussion on the forum.

[Mikuláš Poul]

That is very fair, I guess it is more a feature request then a bug! I also see that atomic is not supported in the writer, would you like me to add the support for atomic while I am there?

[Simon Charette]

I also see that atomic is not supported in the writer, would you like me to add the support for atomic while I am there?

If we're going to add support for run_before, even if unused by the framework itself for now, I think it might be worth making an audit of all documented Migration attributes and adding for them yes.

[Mikuláš Poul]

I've updated my PR to include atomic support as well. After adding run_before and atomic that should be all of the properties defined on Migration (going by ​https://github.com/django/django/blob/cd03e8e2d698e9ccefae2f7687a0400329e6cbe6/django/db/migrations/migration.py#L27-L53)