Make db fields independent of IO

Model.__init__ calls field.get_default() for every field you don't pass:

val = field.get_default()      # django/db/models/base.py, three call sites

So Subject(nickname='x') fires three queries just to construct an object in memory before any save, and three of these defaults use get_or_create:

def default_dataset_type():
    return DatasetType.objects.get_or_create(name='unknown')[0].pk

which means constructing an in-memory Dataset can write to the database. In a DRF create() path, in a transaction that later rolls back, or — during a release — against the wrong database entirely. (Loading from the DB is safe: from_dbuses positional args and skips defaults.)

Triage

For the first four, the value is fixed; the query only exists to guarantee the row exists, and that is a data concern belonging in a fixture or data migration, not in a default. Alyx already ships fixtures for exactly this kind of seed data. In Django 5, db_default is the idiomatic home for the constant cases — it pushes the default into the column so the database supplies it and Python never evaluates anything. It's accepted on related fields.

For default_responsible, the first stock manager can't be a column default, so it moves to where the object is actually created — Subject.save(), or the admin, where BaseAdmin.get_changeform_initial_data already exists for precisely this sort of prefill.

Changing a default alters the field's deconstruction, so each one needs an AlterField migration or makemigrations --check will fail.

Guards

In the Open Alyx 01b_prune script we use:

def _enforce_default_read_only(sender, connection, **kwargs):
    if connection.alias == 'default':
        cursor.execute('SET default_transaction_read_only = ON;')

Extending that to 01a's migrate step would protect against future writes to production during migrate --database public.

Start in django/db/models/base.py at the three field.get_default() call sites, then locate default_species, default_dataset_type, default_data_format, default_source, and default_responsible. Review Subject.save(), BaseAdmin.get_changeform_initial_data, the Open Alyx 01a and 01b scripts, and existing fixtures; done means constants no longer query during construction, dynamic responsibility is handled at creation, seed data is protected, and makemigrations --check passes.