Ensure your migration history is linear.
For a bit of background, see the introductory blog post.
Python 3.6 to 3.9 supported.
Django 2.2 to 3.2 supported.
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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, check the automatic detection of first-party apps. Run this command:
python manage.py create-max-migration-files --dry-run
This command is for creating max_migration.txt
files (more on which later) - in dry run mode it lists the apps it would make such files for.
It tries to automatically detect which apps are first-party, i.e. belong to your project.
The automatic detection checks the path of app’s code to see if is within a virtualenv, but this detection can sometimes fail, for example on editable packages installed with -e
.
If you see any apps listed that aren’t part of your project, define the list of first-party apps’ labels in a FIRST_PARTY_APPS
setting that you combine into INSTALLED_APPS
:
FIRST_PARTY_APPS = [
]
INSTALLED_APPS = FIRST_PARTY_APPS + [
"django_linear_migrations",
...
]
(Note: Django recommends you always list first-party apps first in your project so they can override things in third-party and contrib apps.)
Fourth, create the max_migration.txt
files for your first-party apps by re-running the command without the dry run flag:
python manage.py create-max-migration-files
In the future, when you add a new app to your project, you’ll need to create its max_migration.txt
file.
If you defined FIRST_PARTY_APPS
, add it there, then rerun the creation command for the new app by specifying its label:
python manage.py create-max-migration-files my_new_app
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.
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>
'.
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 rebase-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
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.