Pony ORM with SQLite uses shorthand foreign key syntax instead of explicit

a-test.py

#!/usr/bin/env python

from pony.orm import Database, Optional, Required, Set

db = Database()

@db.on_connect(provider="sqlite")
def pragmas(db, connection):
    cursor = connection.cursor()
    cursor.execute('PRAGMA foreign_keys = true')

db.bind({
    "provider": "sqlite",
    "create_db": "true",
    "filename": "/tmp/pony_test.db"
})

class A(db.Entity):
    _table_ = "A"
    b = Set("B", reverse="a")

class B(db.Entity):
    _table_ = "B"
    a = Optional("A", reverse="b")

db.generate_mapping(create_tables=True)

b-schema-dump.sql

-- The above script produces the following schema.

CREATE TABLE IF NOT EXISTS "A" (
  "id" INTEGER PRIMARY KEY AUTOINCREMENT
);
CREATE TABLE sqlite_sequence(name,seq);
CREATE TABLE IF NOT EXISTS "B" (
  "id" INTEGER PRIMARY KEY AUTOINCREMENT,
  "a" INTEGER REFERENCES "A" ("id") ON DELETE SET NULL
  -- Here I'd expect to see an explicit FOREIGN KEY (...) clause here.
  -- However, see https://sqlite.org/foreignkeys.html
  -- 'Attaching a "REFERENCES <parent-table>" clause to a column definition creates a foreign key constraint'
  -- This is a 'shorthand form to create the foreign key constraint'.
);
CREATE INDEX "idx_b__a" ON "B" ("a");