Django Migrations Add Field with Default as Function of Model

Question

I added a new, non-nullable field to my Django model and am trying to use migrations to deploy that change. How would I set default value to use for existing models to be some function of those models rather than a constant?

As an example let's say I previously had a created_on field and I just added an updated_on field whose value I want to set initially to the model's created_on. How would I do this in a migration?

This is what I am trying to start with:

    migrations.AddField(
        model_name='series',
        name='updated_as',
        field=models.DateTimeField(default=????, auto_now=True),
        preserve_default=False,
    ),

Answer 1

I just learnt how to do this with a single migration!

when running makemigrations django should ask you to set a one-off default. Define whatever you can here to keep it happy, and you'll end up with the migration AddField you mentioned.

migrations.AddField(
    model_name='series',
    name='updated_as',
    field=models.DateTimeField(default=????, auto_now=True),
),

Change this one operation into 3 operations:

1. initially make the field nullable, so the column will be added.
2. call a function to populate the field as needed.
3. alter the field (with AlterField) to make it not nullable (like the above, with no default).

so you end up with something like.

migrations.AddField(
    model_name='series',
    name='updated_as',
    field=models.DateTimeField(null=True, auto_now=True),
),
migrations.RunPython(set_my_defaults, reverse_func),
migrations.AlterField(
    model_name='series',
    name='updated_as',
    field=models.DateTimeField(auto_now=True),
),

with your functions defined as something like:

def set_my_defaults(apps, schema_editor):
    Series = apps.get_model('myapp', 'Series')
    for series in Series.objects.all().iterator():
        series.updated_as = datetime.now() + timedelta(days=series.some_other_field)
        series.save()

def reverse_func(apps, schema_editor):
    pass  # code for reverting migration, if any

Except, you know, not terrible; consider using F expressions and/or database functions to increase migration performance for large databases.