django-safemigrate: Safely run migrations before deployment

django-safemigrate adds a safemigrate command to Django to allow for safely running a migration command when deploying.

Usage

Install django-safemigrate, then add this to the INSTALLED_APPS in the settings file:

INSTALLED_APPS = [
    # ...
    "django_safemigrate",
]

Then mark any migration that may be run during a pre-deployment stage, such as a migration to add a column.

from django_safemigrate import Safe

class Migration(migrations.Migration):
    safe = Safe.before_deploy

At this point you can run the safemigrate Django command to run the migrations, and only these migrations will run. However, if migrations that are not safe to run before the code is deployed are dependencies of this migration, then these migrations will be blocked, and the safemigrate command will fail with an error.

When the code is fully deployed, just run the normal migrate Django command, which still functions normally. For example, you could add the command to the release phase for your Heroku app, and the safe migrations will be run automatically when the new release is promoted.

Safety Options

There are three options for the value of the safe property of the migration.

• Safe.before_deploy

  This migration is only safe to run before the code change is deployed. For example, a migration that adds a new field to a model.

• Safe.after_deploy

  This migration is only safe to run after the code change is deployed. For example, a migration that removes a field from a model.

  By specifying a delay parameter, you can specify when a Safe.after_deploy migration can be run with the safemigrate command. For example, if it's desired to wait a week before applying a migration, you can specify Safe.after_deploy(delay=timedelta(days=7)).

  The delay is used with the datetime when the migration is first detected. The detection datetime is when the safemigrate command detects the migration in a plan that successfully runs. If the migration plan is blocked, such when a Safe.after_deploy is in front of a Safe.before_deploy, no migrations are marked as detected.

  Note that a Safe.after_deploy migration will not run the first time it's encountered.

• Safe.always

  This migration is safe to run before and after the code change is deployed. This is the default that is applied if no safe property is given. For example, a migration that changes the help_text of a field.

Pre-commit Hook

To get the most from django-safemigrate, it is important to make sure that all migrations are marked with the appropriate safe value. To help with this, we provide a hook for use with pre-commit. Install and configure pre-commit, then add this to the repos key of your .pre-commit-config.yaml:

repos:
    -   repo: https://github.com/ryanhiebert/django-safemigrate
        rev: "6.0"
        hooks:
        -   id: check

Nonstrict Mode

Under normal operation, if there are migrations that must run before the deployment that depend on any migration that is marked to run after deployment, the command will raise an error to indicate that there are protected migrations that should have already been run, but have not been, and are blocking migrations that are expected to run.

In development, however, it is common that these would accumulate between developers, and since it is acceptable for there to be downtime during the transitional period in development, it is better to allow the command to continue without raising.

To enable nonstrict mode, add the SAFEMIGRATE setting:

SAFEMIGRATE = "nonstrict"

In this mode safemigrate will run all the migrations that are not blocked by any unsafe migrations. Any remaining migrations can be run after the fact using the normal migrate Django command.

Disabled Mode

To disable the protections of safemigrate entirely, add the SAFEMIGRATE setting:

SAFEMIGRATE = "disabled"

In this mode safemigrate will migrations as if they were using the normal migrate Django command.

Contributing

To get started contributing, you'll want to clone the repository, install dependencies with uv, and set up pre-commit.

git clone [email protected]:ryanhiebert/django-safemigrate.git
cd django-safemigrate
uv sync
pre-commit install

To run the tests use:

uvx --with tox-uv tox

To publish a new version:

1. Find and replace all instances of the previous version with the new version.
2. Commit and push that to origin.
3. Tag the commit with the new version git tag 1.0 and push that to origin.
4. Create the new release on GitHub. It will be published to PyPI automatically.