Populate Your Test Database With Mixins

Maintaining fixtures got you down? Use a mixin!

While Django has support for loading fixtures in its unit testing tools, I have found that maintaining fixtures over time is nothing less than a giant pain in the butt. As your model definitions change over time, remembering to re-run manage.py dumpdata myapp >fixtures.json any time your fixtures are affected is something I guarantee you will forget to do. Not to mention the fact that your database may or may not contain records suitable for testing at any given time. Then all of your unit tests have blown up, it takes you a little bit of time to figure out why, and before you know it you are cursing and out of your flow.

For a time I tried using Alex Gaynor’s django-fixture-generator but that only solved the problem of creating the records you want to test, not the problem of fixtures sitting on disk that are out of sync with your model schema.

After being introduced to Django class-based views’ mixin-heavy style, I cooked up a technique for unit test TestCase classes to create the records they need at runtime, thus bypassing the need for fixtures. In an apps tests/__init__.py I create a PopulateDbMixin:

class PopulateDbMixin(object):
    def populate_db(self):
        """Assume our app has Recipe and Ingredient models"""
        self.models = {}

        self.models['recipes'] = {}
        self.models['recipes']['bread'] = \
            Recipe.objects.create(name='bread')

        self.models['ingredients'] = {}
        self.models['ingredients']['flour'] = \
            Ingredient.objects.create(name='flour')
        self.models['ingredients']['yeast'] = \
            Ingredient.objects.create(name='yeast')

We can now include this mixin in our TestCase classes and run its populate_db method in setUp and have freshly baked bread in each of our test methods:

class DeliciousTestCase(PopulateDbMixin, TestCase):
    def setUp(self):
        self.populate_db()

    def test_can_bake_bread(self):
        bread_recipe = self.models['recipes']['bread']
        yeast = self.models['ingredients']['yeast']

        bread_recipe.ingredients.add(yeast)

        self.assert_(not bread_recipe.goes_horribly_wrong())

The Django TestCase will take care of clearing out the database in between each run; we’re just running populate_db instead of the usual fixture-loading pattern.

This particular technique may be old as time and in use by many teams, but I haven’t seen it written down anywhere so I thought I would share.

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