[Django]-Django — new field: How to set default callable for existing objects

you can do it by manual edit migration,

1. do makemigrations with null
2. do makemigrations with not null
3. Edit first make migration by add datamigration with update and move operations from second migrate file
4. remove the second migrations file,

for example:

from django.db import migrations, models
from django.db.models import F

def set_price_total(apps, schema_editor):
    # Change the myapp on your
    Model = apps.get_model('myapp', 'Model')
    Model.objects.update(price_total=F('price'))


class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0001_initial'),
    ]

    operations = [
        migrations.AddField(
            model_name='model',
            name='price_total',
            field=models.DecimalField(
                decimal_places=2, max_digits=10, null=True),
        ),

        migrations.RunPython(set_price_total),

        migrations.AlterField(
            model_name='model',
            name='price_total',
            field=models.DecimalField(
                decimal_places=2, default=1, max_digits=10),
            preserve_default=False,
        ),
    ]