SQLAlchemy cascade delete with polymorphic tables

poly_delete.py

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, String, Integer, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.orm import sessionmaker

engine = create_engine('sqlite:///:memory:', echo=False)
session = sessionmaker(bind=engine)()
Base = declarative_base()

class Queue(Base):
  __tablename__ = 'queue'
  id    = Column(Integer, primary_key=True)
  tasks = relationship("Task", cascade="all, delete-orphan")
  def __repr__(self):
    return f'{type(self).__name__}({self.id})'

class Task(Base):
  __tablename__ = 'task'
  id    = Column(Integer, primary_key=True)
  type  = Column(String, server_default='base')

  queue_id = Column(Integer, ForeignKey('queue.id'))

  __mapper_args__ = {
    'polymorphic_identity': __tablename__,
    'polymorphic_on': type
  }
  def __repr__(self):
    return f'{type(self).__name__}({self.id}, {self.type})'

class OtherTask(Task):
  __tablename__ = 'otask'
  id   = Column(Integer, ForeignKey('task.id'), primary_key=True)
  level = Column(Integer)

  __mapper_args__ = {
      'polymorphic_identity': __tablename__
  }

class NextTask(Task):
  __tablename__ = 'nexttask'
  id   = Column(Integer, ForeignKey('task.id'), primary_key=True)
  level = Column(Integer)

  __mapper_args__ = {
      'polymorphic_identity': __tablename__
  }

Base.metadata.create_all(engine)

queue = Queue(id=0)
queue.tasks.append(Task(id=0))
queue.tasks.append(OtherTask(id=1, level=9))
queue.tasks.append(NextTask(id=2, level=4))

session.add(queue)
session.commit()

for q in session.query(Queue):
  print(q)
  for task in q.tasks:
    print(task)

print(session.query(Task).all())
session.delete(queue)
session.commit()
print(session.query(Task).all())