A while ago, I wrote my first Django migration that actually mattered, and was a data migration rather than a nice, simple, auto generated one for adding columns. I am of the view that migrations are scary. Especially data migrations, where you're shuffling data around in your tables. If you're scared about something being wrong, then right a test; and so I found this.

The post describes a TestCase subclass that sets the database to a starting migration state, lets you add some data, run the migrations through to a new state, then assert things about how the data now looks. This seems like an excellent way to gain some confidence that your migrations mutates your data in the correct way. However, the linked article assumes South migrations are being used, so is no help for Django>=1.7, which is why I wrote django_migration_test, which aims to work with Django's new migrations framework.

I'm keen to find out if it actually works in various edge cases and database engines. I also want to know if this is actually a solved problem, or if this is not the best way of testing in this case. It's on PyPI, and you can install it with

pip install django-migration-test

and then write a test that looks a bit like:

from django_migration_test import MigrationTest


class MyMigrationTest(MigrationTest):

    # At present, we can only run migrations for one app at a time.
    app_name = 'my_app'
    # At present, these need to be full names of migrations, not just
    # prefixes.
    before = '0001_initial'
    after = '0002_change_fields'

    # Can have any name, is just a test method. MigrationTest
    # subclasses django.test.TestCase
    def test_migration(self):
        # Load some data. Don't directly import models. At this point,
        # the database is at self.before, and the models have fields
        # set accordingly.

        MyModel = self.get_model_before('MyModel')

        # ... save some models

        # Trigger the migration
        self.run_migration()

        # Now run some assertions based on what the data should now
        # look like. The database will now be at self.after. To run
        # queries via the models, reload the model.

        MyModel = self.get_model_after('MyModel')