Custom operations in migrations

migrations.py

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import models, migrations


def make_balance_nullable_forwards(apps, schema_editor):
    Carrier = apps.get_model("carriers", "Carrier")
    default_balance_carriers = Carrier.objects.filter(balance=0)
    default_deposit_carriers = Carrier.objects.filter(deposit=0)
    for carrier in default_balance_carriers:
        carrier.balance = None
        carrier.save()
    for carrier in default_deposit_carriers:
        carrier.deposit = None
        carrier.save()


def make_balance_nullable_reverse(apps, schema_editor):
    Carrier = apps.get_model("carriers", "Carrier")
    null_balance_carriers = Carrier.objects.filter(balance__isnull=True)
    null_deposit_carriers = Carrier.objects.filter(deposit__isnull=True)
    for carrier in null_balance_carriers:
        carrier.balance = 0
        carrier.save()
    for carrier in null_deposit_carriers:
        carrier.deposit = 0
        carrier.save()


class Migration(migrations.Migration):

    dependencies = [
        ('carriers', '0004_auto_20151008_0641'),
    ]

    operations = [
        migrations.AddField(
            model_name='carrier',
            name='currency',
            field=models.CharField(default=b'DOL', max_length=3, choices=[(b'DOL', b'Dollar'), (b'EUR', b'Euro'), (b'POU', b'Pounds')]),
            preserve_default=True,
        ),
        migrations.AddField(
            model_name='carrier',
            name='notes',
            field=models.TextField(null=True, blank=True),
            preserve_default=True,
        ),
        migrations.AddField(
            model_name='carrier',
            name='short_name',
            field=models.CharField(max_length=8, unique=True, null=True, blank=True),
            preserve_default=True,
        ),
        migrations.AlterField(
            model_name='carrier',
            name='balance',
            field=models.IntegerField(null=True, blank=True),
        ),
        migrations.AlterField(
            model_name='carrier',
            name='deposit',
            field=models.PositiveIntegerField(null=True, blank=True),
        ),
        migrations.RunPython(make_balance_nullable_forwards, make_balance_nullable_reverse),

    ]