Truncating an Alembic migrations history

published 2019-06-01

In projects that use SQLAlchemy and Alembic [1] via Flask-Migrate [2], you may want to truncate the migrations history. By that I mean: rewrite all the migrations up to some point as a single initial migration, to avoid replaying them every single time you create a new database instance. Of course, you only want to do that if you have already migrated all your database instances at least up to that point.

As far as I know, there is no Alembic feature to do this automatically. However, I found a way to avoid having to write the migration by hand. Here is an example of how you can achieve this with a project using Git, PostgreSQL, and environment variables for configuration [3].

First, checkout a commit of your project where the first migration you want to keep is the current migration, and create a temporary branch. Then, take a note of the ID of that migration (for instance `abcd12345678`), delete the whole `migrations` directory and reinitialize Alembic.

git checkout $my_commit
git checkout -b tmp-alembic
rm -rf migrations
flask db init

At this point, using Git, revert changes to files where you should keep your changes, such as `script.py.mako` and `env.ini`. Then, create a temporary empty database to work with.

git checkout migrations/script.py.mako
git checkout migrations/env.py
createdb -T template0 my-temp-db

Now create the initial migration that corresponds to your model, with the ID that you noted previously, e.g.:

MY_DATABASE_URI="postgresql://postgres@localhost/my-empty-db" \
    flask db migrate --rev-id abcd12345678

Finally, you can delete the temporary database, commit your changes to your temporary branch, merge it into your main development branch and delete it:

dropdb my-empty-db
git commit
git checkout dev
git merge tmp-alembic
git branch -D tmp-alembic

1: https://alembic.sqlalchemy.org

2: https://flask-migrate.readthedocs.io

3: https://12factor.net

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