django-linear-migrations 1.1.0

Ensure your migrations are linear.

Ensure your migration history is linear.

For a bit of background, see the introductory blog post.

Requirements

Python 3.6 to 3.9 supported.

Django 2.2 to 3.1 supported.

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Installation

First, install with pip:

python -m pip install django-linear-migrations

Second, add the app to your INSTALLED_APPS setting:

INSTALLED_APPS = [
    ...
    "django_linear_migrations",
    ...
]

The app relies on overriding the built-in makemigrations command. If your project has a custom makemigrations command, ensure the app containing your custom command is above django_linear_migrations, and that your command subclasses its Command class:

# myapp/management/commands/makemigrations.py
from django_linear_migrations.management.commands.makemigrations import (
    Command as BaseCommand,
)


class Command(BaseCommand):
    ...

Third, run this one-off command for installation:

python manage.py create-max-migration-files

This creates a new max_migration.txt file in each of your first-party apps’ migrations directories and exits. More on those files below…

Usage

django-linear-migrations helps you work on Django projects where several branches adding migrations may be in progress at any time. It enforces that your apps have a linear migration history, avoiding merge migrations and the problems they can cause from migrations running in different orders. It does this by making makemigrations record the name of the latest migration in per-app max_migration.txt files. These files will then cause a merge conflicts in your source control tool (Git, Mercurial, etc.) in the case of migrations being developed in parallel. The first merged migration for an app will prevent the second from being merged, without addressing the conflict. The included rebase-migration command can help automatically such conflicts.

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>

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 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 can start to fix the conflict by pulling your latest main branch, then rebasing your titles branch on top of it. When you do this, Git will report the conflict:

$ git switch main
$ git pull
...
$ 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 conflcited 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

Note this 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 other parts of the source code as well, such as in the models.

Inspiration

I’ve seen similar techniques to the one implemented by django-linear-migrations at several places, and they acted as the inspiration for putting this package together. My previous client Pollen and current client ev.energy both have implementations. This Doordash blogpost covers a similar system that uses a single file for tracking latest migrations. And there’s also a package called django-migrations-git-conflicts which works fairly similarly.