Temporary models in Django

Occasionally I need to create a temporary model within a Django application.

The most recent occasion for this was a one-off management command I was writing to import some data from a legacy system. The old database, for some reason, eschewed foreign keys in favour of char fields in a linking table which referred to the relevant rows. In converting this to a Django app, and wanting to use sensible database structure, I planned to replace this with normal ForeignKey fields. But I needed to temporarily hold onto the old references during the import process, so that I could set the new FK properly.

I didn't want to add a field to my model, create a migration for the new field, do the import, then add another migration to drop the field again, so a quick answer was to create a temporary table to hold the linking data during the import. And I wanted to define it within the management command itself, again so as not to pollute the real models with temporary code.

Surprisingly, this turned out to be quite easy. Here's the code:

from django.db import models, cursor
from django.contrib.contenttypes.management import update_contenttypes
from django.core.management import call_command

class TempCustomerAddress(models.Model):
    address = models.ForeignKey('accounts.Address')
    legacy_id = models.CharField(max_length=12, unique=True)

    class Meta:
        app_label = 'utils'

class Command(NoArgsCommand):

    def handle_noargs(self, **options):
        models.register_models('utils', TempCustomerAddress)

        # ... do importing and stuff referring to TempCustomerAddress ...

        cursor = connection.cursor()
        cursor.execute('DROP TABLE `utils_tempcustomeraddress`')

Firstly I define the model, giving it an explicit app_label referring to an existing application within my project - not, incidentally, the one containing the actual command.

Then within the body of the command, it was pretty much just a matter of registering the model and running syncdb. It turns out that there is a very simple, although undocumented, function, register_models, to do this - you just need to pass it the name of the application to register the model into, and the model class itself. Again, I'm using the 'utils' app to register the model against - mainly because in our project that isn't managed by South, so syncdb will work. One thing I did have to do though was disconnect the post_syncdb signal which creates content types, as this seemed not to like the temporary model.

The final task, after the import had run, was to drop the temporary table. Since I'm not using south here I have to do that manually by running some SQL.

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