django-fixture-magic: Testing issues with real data.

I just released Fixture Magic.

When dealing with legacy data, you’ll run into all kinds of edge cases. Perhaps, an object might not display correctly unless it has the right parameters, or if it has null parameters it might not display at all. So when testing Django, it’s nice to actually use non-dummy data.

Luckily Django has a way of pulling real data out of your database using django.core.serializers:

from addons.models import Addon
a = Addon.objects.get(id=3615)
from django.core.serializers import serialize
jsonize = lambda a: serialize("json", a, indent=4)
jsonize([a])

This solution runs well in a Django shell and can be lots of fun for the whole family… until things get complicated. ### Serializing alone isn’t enough.

Serializing a fixture with foreign keys means you’ll have an un-loadable fixture unless you serialize the dependent fixtures. Even for one or two foreign keys, this can be a pain. For addons.mozilla.org, we have a spidery-web of dependencies: Files need a Version which needs an Addon which need Translations.

Thus begat the dump_object management command. Give it an app, model name and a pk and it will give you not only a serialized JSON of that object, but all the objects that it requires.

Example:

./manage.py dump_object files.file 64874 64876 > my_new_fixture.json

This looks for the File model in the files app and pulls out of the database Files instances with pks of 64874 and 64876. It then recursively searches for any required objects.

Too much serial

If you create a lot of fixtures, you’ll eventually have overlapping serialized objects. In addons.mozilla.org we have Addons, Versions (which depend on Addons) and AddonCategorys (which depend on Addons and Categorys). If we wanted to get serialize a specific Addon, it’s dependent Versions and AddonCategorys it makes sense to start with dump_objecting the related Version and then dump_objecting the AddonCategory. Both dump_object commands will fetch the Addon in question, resulting in duplicated data.

To combat this we can use merge_fixtures to dedupe our fixtures:

./manage.py dump_object versions.version 64874 > 1.json
./manage.py dump_object categories.addoncategory > 2.json
./manage.py merge_json 1.json 2.json > happy_fixture.json

This should make creating test data slightly less painful. So give it a try.