Mapping multiple classes to a single table in a mapped inheritance hierarchy does not populate tables correctly

The following code:

from sqlalchemy import *
from sqlalchemy.orm import *

db = create_engine('sqlite:///sa.db')

db.echo = False  # Try changing this to True and see what happens

metadata = MetaData(db)

class Task(object):
  pass


class CustomTask(Task):
  pass


class MyTask(CustomTask):
  pass

task = Table('task', metadata,
    Column('task_id', Integer, primary_key=True),
    Column('type', String(64)),
    )

custom_task = Table('task_custom', metadata,
    Column('task_id', Integer, ForeignKey('task.task_id'), primary_key=True),
    )

mapper(
    Task, task,
    polymorphic_on=task.c.type,
    polymorphic_identity='Task',
    select_table=custom_task.outerjoin(task),
    )

mapper(
    CustomTask, custom_task,
    polymorphic_identity='CustomTask',
    inherits=Task,
    )

mapper(
    MyTask, custom_task,
    polymorphic_identity='MyTask',
    inherits=CustomTask,
    )

metadata.create_all()

Session = sessionmaker(autoflush=True, transactional=True)
Session.configure(bind=db)

session = Session()

task = Task()
rtask = CustomTask()
crtask = MyTask()

session.save_or_update(task)
session.save_or_update(rtask)
session.save_or_update(crtask)
session.commit()

session.flush()

for task in session.query(task):
  print task

Produces this SQL:

BEGIN TRANSACTION;
CREATE TABLE task (
    task_id INTEGER NOT NULL, 
    type VARCHAR(64), 
    PRIMARY KEY (task_id)
);
INSERT INTO "task" VALUES(1,'Task');
INSERT INTO "task" VALUES(2,'CustomTask');
CREATE TABLE task_custom (
    task_id INTEGER NOT NULL, 
    PRIMARY KEY (task_id), 
     FOREIGN KEY(task_id) REFERENCES task (task_id)
);
INSERT INTO "task_custom" VALUES(2);
INSERT INTO "task_custom" VALUES(3);
COMMIT;

Note the dangling task_custom row with task_id=3.

Comments (2)