Changing the primary key of a model in Django

I had to change the primary key of a django model, and I wanted to create a migration for this.

The previous model was using django automatic primary key fields

I firstly changed the model to include the new uuid field, and added the id field (the old primary key), like this:

    uuid = models.UUIDField(
        unique=True, primary_key=True, default=uuid.uuid4, editable=False
    )
    id = models.IntegerField(null=True, blank=True)

Then I created the migration, it:

Adds a new uuid field/column in the database
Iterate over the existing items in the table, and generates an uuid for them
Change the old primary key to a different type
Drop the old index
Mark the new uuid as a primary key.

To generate the migrations I did django-admin makemigrations, and iterated on it. Here is the migration I ended up with:

import uuid

from django.db import migrations, models

class Migration(migrations.Migration):
    dependencies = [
        ("umap", "0017_migrate_to_openstreetmap_oauth2"),
    ]

    operations = [
        # Add the new uuid field
        migrations.AddField(
            model_name="datalayer",
            name="uuid",
            field=models.UUIDField(
                default=uuid.uuid4, editable=False, null=True, serialize=False
            ),
        ),
        # Generate UUIDs for existing records
        migrations.RunSQL("UPDATE umap_datalayer SET uuid = gen_random_uuid()"),
        # Remove the primary key constraint
        migrations.RunSQL("ALTER TABLE umap_datalayer DROP CONSTRAINT umap_datalayer_pk"),
        # Drop the "id" primary key…
        migrations.AlterField(
            "datalayer", name="id", field=models.IntegerField(null=True, blank=True)
        ),
        # … to put it back on the "uuid"
        migrations.AlterField(
            model_name="datalayer",
            name="uuid",
            field=models.UUIDField(
                default=uuid.uuid4,
                editable=False,
                unique=True,
                primary_key=True,
                serialize=False,
            ),
        ),
    ]

Generating UUIDs in pure python

The uuid generation can also be done with pure python, like this. It works with all databases, but might be slower. Use it with migrations.RunPython().

def gen_uuid(apps, schema_editor):
    DataLayer = apps.get_model("umap", "DataLayer")
    for row in DataLayer.objects.all():
        row.uuid = uuid.uuid4()
        row.save(update_fields=["uuid"])

Getting the constraint name

One of the things that took me some time is to have a way to get the constraint name before removing it. I wanted to do this with the Django ORM, but I didn't find how. So here is how in plain SQL. This only works with PostgreSQL, though.

migrations.RunSQL("""
DO $$
BEGIN
    EXECUTE 'ALTER TABLE umap_datalayer DROP CONSTRAINT ' || (
        SELECT indexname
        FROM pg_indexes
        WHERE tablename = 'umap_datalayer' AND indexname LIKE '%pkey'
    );
END $$;
"""),