django-test-migrations

Test django schema and data migrations, including migrations' order and best practices.

Features

  • Allows to test django schema and data migrations
  • Allows to test both forward and rollback migrations
  • Allows to test the migrations order
  • Allows to test migration names
  • Allows to test database configuration
  • Fully typed with annotations and checked with mypy, PEP561 compatible
  • Easy to start: has lots of docs, tests, and tutorials

Read the announcing post.
See real-world usage example.

Installation

pip install django-test-migrations

We support several django versions:

  • 1.11
  • 2.2
  • 3.0
  • 3.1

Other versions might work too, but they are not officially supported.

Testing django migrations

Testing migrations is not a frequent thing in django land.
But, sometimes it is totally required. When?

When we do complex schema or data changes
and what to be sure that existing data won't be corrupted.
We might also want to be sure that all migrations can be safely rolled back.
And as a final touch we want to be sure that migrations
are in the correct order and have correct dependencies.

Testing forward migrations

To test all migrations we have a Migrator class.

It has three methods to work with:

  • .apply_initial_migration() which takes app and migration names to generate
    a state before the actual migration happens. It creates the before state
    by applying all migrations up to and including the ones passed as an argument.

  • .apply_tested_migration() which takes app and migration names to perform the
    actual migration

  • .reset() to clean everything up after we are done with testing

So, here's an example:

from django_test_migrations.migrator import Migrator

migrator = Migrator(database='default')

# Initial migration, currently our model has only a single string field:
# Note:
# We are testing migration `0002_someitem_is_clean`, so we are specifying
# the name of the previous migration (`0001_initial`) in the
# .apply_initial_migration() method in order to prepare a state of the database
# before applying the migration we are going to test.
#
old_state = migrator.apply_initial_migration(('main_app', '0001_initial'))
SomeItem = old_state.apps.get_model('main_app', 'SomeItem')

# Let's create a model with just a single field specified:
SomeItem.objects.create(string_field='a')
assert len(SomeItem._meta.get_fields()) == 2  # id + string_field

# Now this migration will add `is_clean` field to the model:
new_state = migrator.apply_tested_migration(
    ('main_app', '0002_someitem_is_clean'),
)
SomeItem = new_state.apps.get_model('main_app', 'SomeItem')

# We can now test how our migration worked, new field is there:
assert SomeItem.objects.filter(is_clean=True).count() == 0
assert len(SomeItem._meta.get_fields()) == 3  # id + string_field + is_clean

# Cleanup:
migrator.reset()

That was an example of a forward migration.

Backward migration

The thing is that you can also test backward migrations.
Nothing really changes except migration names that you pass and your logic:

migrator = Migrator()

# Currently our model has two field, but we need a rollback:
old_state = migrator.apply_initial_migration(
    ('main_app', '0002_someitem_is_clean'),
)
SomeItem = old_state.apps.get_model('main_app', 'SomeItem')

# Create some data to illustrate your cases:
# ...

# Now this migration will drop `is_clean` field:
new_state = migrator.apply_tested_migration(('main_app', '0001_initial'))

# Assert the results:
# ...

# Cleanup:
migrator.reset()

Testing migrations ordering

Sometimes we also want to be sure that our migrations are in the correct order.
And all our dependecies = [...] are correct.

To achieve that we have plan.py module.

That's how it can be used:

from django_test_migrations.plan import all_migrations, nodes_to_tuples

main_migrations = all_migrations('default', ['main_app', 'other_app'])
assert nodes_to_tuples(main_migrations) == [
    ('main_app', '0001_initial'),
    ('main_app', '0002_someitem_is_clean'),
    ('other_app', '0001_initial'),
    ('main_app', '0003_update_is_clean'),
    ('main_app', '0004_auto_20191119_2125'),
    ('other_app', '0002_auto_20191120_2230'),
]

This way you can be sure that migrations
and apps that depend on each other will be executed in the correct order.

Test framework integrations 🐍

We support several test frameworks as first-class citizens.
That's a testing tool after all!

Note that the Django post_migrate signal's receiver list is cleared at
the start of tests and restored afterwards. If you need to test your
own post_migrate signals then attach/remove them during a test.

pytest

We ship django-test-migrations with a pytest plugin
that provides two convinient fixtures:

  • migrator_factory that gives you an opportunity
    to create Migrator classes for any database
  • migrator instance for the 'default' database

That's how it can be used:

import pytest

@pytest.mark.django_db()
def test_pytest_plugin_initial(migrator):
    """Ensures that the initial migration works."""
    old_state = migrator.apply_initial_migration(('main_app', None))

    with pytest.raises(LookupError):
        # Models does not yet exist:
        old_state.apps.get_model('main_app', 'SomeItem')

    new_state = migrator.apply_tested_migration(('main_app', '0001_initial'))
    # After the initial migration is done, we can use the model state:
    SomeItem = new_state.apps.get_model('main_app', 'SomeItem')
    assert SomeItem.objects.filter(string_field='').count() == 0

unittest

We also ship an integration with the built-in unittest framework.

Here's how it can be used:

from django_test_migrations.contrib.unittest_case import MigratorTestCase

class TestDirectMigration(MigratorTestCase):
    """This class is used to test direct migrations."""

    migrate_from = ('main_app', '0002_someitem_is_clean')
    migrate_to = ('main_app', '0003_update_is_clean')

    def prepare(self):
        """Prepare some data before the migration."""
        SomeItem = self.old_state.apps.get_model('main_app', 'SomeItem')
        SomeItem.objects.create(string_field='a')
        SomeItem.objects.create(string_field='a b')

    def test_migration_main0003(self):
        """Run the test itself."""
        SomeItem = self.new_state.apps.get_model('main_app', 'SomeItem')

        assert SomeItem.objects.count() == 2
        assert SomeItem.objects.filter(is_clean=True).count() == 1

Choosing only migrations tests

In CI systems it is important to get instant feedback. Running tests that
apply database migration can slow down tests execution, so it is often a good
idea to run standard, fast, regular unit tests without migrations in parallel
with slower migrations tests.

pytest

django_test_migrations adds migration_test marker to each test using
migrator_factory or migrator fixture.
To run only migrations test, use -m option:

pytest -m migration_test  # runs only migraion tests
pytest -m "not migration_test"  # runs all except migraion tests

unittest

django_test_migrations adds migration_test
tag
to every MigratorTestCase subclass.
To run only migrations tests, use --tag option:

python mange.py test --tag=migration_test  # runs only migraion tests
python mange.py test --exclude-tag=migration_test  # runs all except migraion tests

Django Checks

django_test_migrations comes with 2 groups of Django's checks for:

  • detecting migrations scripts automatically generated names
  • validating some subset of database settings

Testing migration names

django generates migration names for you when you run makemigrations.
And these names are bad (read more about why it is bad)!
Just look at this: 0004_auto_20191119_2125.py

What does this migration do? What changes does it have?

One can also pass --name attribute when creating migrations, but it is easy to forget.

We offer an automated solution: django check
that produces an error for each badly named migration.

Add our check into your INSTALLED_APPS:

INSTALLED_APPS = [
    # ...

    # Our custom check:
    'django_test_migrations.contrib.django_checks.AutoNames',
]

And then in your CI run:

python manage.py check --deploy

This way you will be safe from wrong names in your migrations.

Do you have a migrations that cannot be renamed? Add them to the ignore list:

# settings.py

DTM_IGNORED_MIGRATIONS = {
    ('main_app', '0004_auto_20191119_2125'),
    ('dependency_app', '0001_auto_20201110_2100'),
}

And we won't complain about them.

Or you can completely ignore entire app:

# settings.py

DTM_IGNORED_MIGRATIONS = {
    ('dependency_app', '*'),
    ('another_dependency_app', '*'),
}

Database configuration

Add our check to INSTALLED_APPS:

INSTALLED_APPS = [
    # ...

    # Our custom check:
    'django_test_migrations.contrib.django_checks.DatabaseConfiguration',
]

Then just run check management command in your CI like listed in section
above.

GitHub

https://github.com/wemake-services/django-test-migrations