OnDeleteType Literal is missing SET DEFAULT and NO ACTION — rejects valid SQL referential actions #1836

Description

The OnDeleteType type alias in sqlmodel/main.py (line 91) is defined as:

OnDeleteType = Literal["CASCADE", "SET NULL", "RESTRICT"]

The SQL standard defines 5 referential actions for ON DELETE / ON UPDATE:

1. CASCADE
2. SET NULL
3. SET DEFAULT
4. RESTRICT
5. NO ACTION

SQLAlchemy's ForeignKey(ondelete=...) accepts any string and its documentation explicitly lists all 5 as valid values.

Impact

• NO ACTION is the default behavior in PostgreSQL and most databases. Developers who want to be explicit in their schema (common in migration-driven workflows with Alembic) cannot express this without type errors.
• SET DEFAULT is used when a foreign key column has a server_default and the parent row is deleted. This is supported by PostgreSQL, MySQL, and SQLite.

A developer hitting this gets a confusing mypy/pyright error on a perfectly valid schema definition. The only workarounds are casting to Any or dropping down to sa_column.

Fix

OnDeleteType = Literal["CASCADE", "SET NULL", "SET DEFAULT", "RESTRICT", "NO ACTION"]

One-line change.