Gotchas about Fixtures for Initial Data

One part of Django testing environment is fixtures. They are pre-prepared datasets to fill into the database before running tests. Django uses a separate database for running tests, but the fixtures from the files initial_data.* are also loaded into the main database when you synchronize it.

I make fixtures for my application like this:

python manage.py dumpdata --format=json --indent=4 myapp > apps/myapp/fixtures/initial_data.json 

The initial data is read out from apps/myapp/fixtures/initial_data.json and written to the main or the test database when I synchronize the database

python manage.py syncdb

or when I run the tests

python manage.py test myapp

Fixtures are great for deploying standard data like predefined categories, lists of countries and languages, default flatpages, default navigation, default user groups, and so on. However, you should be very cautious with them while developing.

When I create new models, it's common practice to me to sync db so that the new database tables are created and the database schema reflects the models. Just after creation of new tables all fixtures called initial_data from all applications will be loaded. N.B. The fixtures from initial_data overwrite all existing data while synchronizing database. So if you have some important data that differs from the defaults, better make some database backup before syncing or use sql command to print out the SQL statements and execute them for the database manually:

python manage.py sql myapp_with_new_models

You might have pre_save signal handlers or custom save methods (check an example below) which should recognize newly created objects and do something special with them, i.e. prepare PDF reports, generate images, send emails, index for global text search, or something else. Usually in such cases I checked the existence of the primary key: the object is new if it has no primary key. But this is wrong when you use fixtures, because fixtures come with primary keys. N.B. The object is new only if there is no object in the database which primary key equals to the primary key of the current object.

class MyModel(models.Model):
    ...
    def save(self, *args, **kwargs):
        is_new = True
        pk = self._get_pk_val()
        model = type(self)
        if pk and model._default_manager.filter(pk=pk):
            is_new = False
        # something before saving
        super(model, self).save(*args, **kwargs)
        # something after saving

aka

class MyModel(models.Model):
    ...
    def save(self, *args, **kwargs):
        is_new = True
        if self.id and MyModel.objects.filter(id=self.id):
            is_new = False
        # something before saving
        super(MyModel, self).save(*args, **kwargs)
        # something after saving

Another alternative for storing default data would be custom sql located at apps/myapp/sql/mymodel.sql, but I haven't tried that yet and probably won't.

BTW, happy programmer day!