django-linear-migrations 1.4.0

System Checks

django-linear-migrations comes with several system checks that verify that your max_migration.txt files are in sync. These are:

• dlm.E001: <app_label>’s max_migration.txt does not exist.

• dlm.E002: <app_label>’s max_migration.txt contains multiple lines.

• dlm.E003: <app_label>’s max_migration.txt points to non-existent migration ‘<bad_migration_name>’.

• dlm.E004: <app_label>’s max_migration.txt contains ‘<max_migration_name>’, but the latest migration is ‘<real_max_migration_name>’.

rebase-migration command

This management command can help you fix migration conflicts. Following a conflicted “rebase” operation in your source control tool, run it with the name of the app to auto-fix the migrations for:

$ python manage.py rebase-migration <app_label>

Note rebasing the migration might not always be the correct thing to do. If the migrations in main and feature branches have both affected the same models, rebasing the migration on the end may not make sense. However, such parallel changes would normally cause conflicts in your models files or other parts of the source code as well.

Let’s walk through an example using Git, although it should extend to other source control tools.

Imagine you were working on your project’s books app in a feature branch called titles and created a migration called 0002_longer_titles. Meanwhile a commit has been merged to your main branch with a different 2nd migration for books called 0002_author_nicknames. Thanks to django-linear-migrations, the max_migration.txt file will show as conflicted between your feature and main branches.

You start the fix by reversing your new migration from your local database. This is necessary since it will be renamed after rebasing and seen as unapplied. You do this by switching to the feature branch titles migrating back to the last common migration:

$ git switch titles
$ python manage.py migrate books 0001

You then fetch the latest code:

$ git switch main
$ git pull
...

You then rebase your titles branch on top of it, for which Git will detect the conflict on max_migration.txt:

$ git switch titles
$ git rebase main
Auto-merging books/models.py
CONFLICT (content): Merge conflict in books/migrations/max_migration.txt
error: could not apply 123456789... Increase Book title length
Resolve all conflicts manually, mark them as resolved with
"git add/rm <conflicted_files>", then run "git rebase --continue".
You can instead skip this commit: run "git rebase --skip".
To abort and get back to the state before "git rebase", run "git rebase --abort".
Could not apply 123456789... Increase Book title length

If you look at the contents of the books app’s max_migration.txt at this point, it will look something like this:

$ cat books/migrations/max_migration.txt
<<<<<<< HEAD
0002_author_nicknames
=======
0002_longer_titles
>>>>>>> 123456789 (Increase Book title length)

It’s at this point you can use rebase-migration to automatically fix the books migration history:

$ python manage.py rebease-migration books
Renamed 0002_longer_titles.py to 0003_longer_titles.py, updated its dependencies, and updated max_migration.txt.

This places the conflicted migration on the end of the migration history. It renames the file appropriately, modifies its dependencies = [...] declaration, and updates the migration named in max_migration.txt appropriately.

After this, you should be able to continue the rebase:

$ git add books/migrations
$ git rebase --continue

And then migrate your local database to allow you to continue development:

$ python manage.py migrate books
Operations to perform:
  Target specific migration: 0003_longer_titles, from books
Running migrations:
  Applying books.0002_author_nicknames... OK
  Applying books.0003_longer_titles... OK