django-dbdiff

I'm pretty lazy when it comes to writing tests for existing code, however, I'm even lazier when it comes to repetitive manual testing action.

This package aims at de-duplicating the data import tests from django-representatives and django-representatives-votes which is to be re-used in django-cities-light.

Database state assertion

A nice way to test a data import script is to create a source data fixture with a subset of data, ie. with only 10 cities instead of 28K or only 3 european parliament representatives instead of 3600, feed the import function with that and then compare the database state with a django fixture. For example:

• use such a command to create a small data extract shuf -n3 cities15000.txt > cities_light/tests/cities_test_fixture.txt,
• use it against the import script on a clean database,
• verify the database manually, and run django-admin dumpdata --indent=4 cities_light > cities_light/tests/cities_test_expected.txt
• then, make a test case that calls the import script against the fixture and call test_light's function to assert that the database contains only the expected data.

When a bug is fixed, just add the case to the fixture and repeat the process to create new expected data dumps, use coverage to ensure no case is missed.

Predictible serialization

It is important to use serializers which dump data in a predictible way because this app relies on diff between an expected - user-generated and versioned - fixture and dumped database data.

Django's default model-to-dict logic - implemented in django.core.serializers.python.Serializer.get_dump_object() - returns a dict, this app registers a slightly modified version of the default json serializer which returns OrderedDicts instead.

In addition, dbdiff serialization forces Decimal normalization, because trailing zeros could happen in inconsistent ways.

Cross-database fixture compatibility

MySQL doesn't have microseconds in datetimes, so dbdiff's serializer removes microseconds from datetimes so that fixtures are cross-database compatible which make them usable for cross-database testing.

Usage

MySQL, SQLite and PostgreSQL, Python 2.7 and 3.4 are supported along with Django 1.7 to 1.10 - it's always better to support django's master so that we can upgrade easily when it is released.

Install django-dbdiff with pip and add dbdiff to INSTALLED_APPS.

When dbdiff is installed, dumpdata will use its serializers which have predictible output and cross-database support, so fixtures dumped without dbdiff installed will have to be regenerated after dbdiff is installed.

Example:

from dbdiff import dbdiff

your_import_function()
assert not dbdiff.diff('your_app/tests/some_fixture.json')

If any difference is found between the database and the test fixture, then diff() will return the diff as outputed by GNU diff.

A context manager that will raise an exception if a diff is found is also provided, it's able to delete models and reset the PK sequences for them:

with dbdiff.exact('your_app/fixture.json'):
    # do stuff

More public API tests can be found in dbidff/tests/test_dbdiff.py.

Django model observer

It is interresting to note that a related, perhaps sort-of similar app exists: https://github.com/Griffosx/djmo