matutter · January 26, 2020 05:30 · Jan 26, 2020
diff --git a/poly_delete.py b/poly_delete.py
@@ -0,0 +1,68 @@
+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())