How to call a static methods on a django model class during a south migration

Question

I’m writing a data migration in south to fix some denormalized data I screwed up in earlier code. The way to figure out the right value for the incorrect field is to call a static method on the django model class. The code looks like this:

class Account(models.Model):
    name = models.CharField()

    @staticmethod
    def lookup_by_name(name):
        # There's actually more to it than this
        return Account.objects.get(name=name)

class Record(models.Model):
    account_name = models.CharField()
    acct = models.ForeignKey('Account')

…

class Migration(DataMigration):

    def forwards(self, orm):
        # Fixing Records with the wrong FK to Account
        for record in orm.Record.objects.all():
            record.acct = orm.Account.lookup_by_name(record.account_name)
            record.save()

But this fails with

AttributeError: type object 'Account' has no attribute 'lookup_by_name'

I’m guessing south just doesn’t support @staticmethods on model classes?

Trying to import Account directly fails, unless I also import Record directly and completely ignore the ORM object. Is that a safe option, since it’s a data migration and the schema isn’t changing? Or should I just run this fix by hand rather than in the context of a south migration.

Asked By: Leopd

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Source

Answer 1

Aren’t you using different names lookup_by_name and lookup_name?

Answered By: dzida

Answer 2

You can’t use methods from models.py in south migrations. The reason is that in the future models.py will evolve and sooner or later you will delete those methods, then migration will be broken.

You should put all code needed by migration in migration file itself.

Answered By: Tomasz Wysocki

Answer 3

Here’s the pertinent section of the South docs explaining why your methods don’t work:

Rationale behind the serialisation

Answered By: Jason

How to call a static methods on a django model class during a south migration

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