Convert data on AlterField django migration

Question

I have a production database and need to keep safe the data. I want to change a Field in model and convert all data inside that database with this change.

Old field<

Answer 1

In the past, I've always done this with the help of Djangos RunPython operation. Create a custom migration that handles the following.

1. Change name of field.
2. Add new field of desired type.
3. RunPython to handle the logic of converting from one to the other.
4. Delete old field.

def migrate_time_to_positive_int(apps, schema_editor):

    MyModel = apps.get_model('myapp', 'MyModel')

    for mm in MyModel.objects.all():

        field_old_time = mm.field_name_old
        field_new_int = field_old_time.total_seconds() / 60
        mm.field_name = field_new_int 
        mm.save()

class Migration(migrations.Migration):

    operations = [
        migrations.RenameField(
            model_name='mymodel',
            old_name='field_name',
            new_name='field_name_old',
        ),
        migrations.AddField(
            model_name='mymodel',
            name='field_name',
            field=models.PositiveIntegerField(),
        ),
        migrations.RunPython(migrate_time_to_positive_int),
        migrations.RemoveField(
            model_name='mymodel',
            name='field_name_old',
        )
    ]

field_name_old.total_seconds() / 60 might need to be adjusted, but you get the idea.