OK, thanks for the reply Mike! On Thursday, September 4, 2014 8:45:24 AM UTC-7, Michael Bayer wrote: > > this is probably more of a Pyramid question. > > I’m pretty allergic to traversal myself :) > > > On Sep 3, 2014, at 2:58 PM, Milo Toor <milo...@gmail.com <javascript:>> > wrote: > > Hi. > > I am trying to wrap a polymorphic model so that it can act as a traversal > node in a Pyramid application: > > from sqlalchemy import Column, Integer, String, create_engine > from sqlalchemy.orm import sessionmaker > from sqlalchemy.ext.declarative import declarative_base > > > Base = declarative_base() > engine = create_engine('sqlite:///:memory:', echo=True) > Session = sessionmaker(bind=engine) > > > # This is the class we wish to wrap > class Employee(Base): > __tablename__ = 'employee' > id = Column(Integer, primary_key=True) > name = Column(String(50)) > type = Column(String(20)) > > > __mapper_args__ = { > 'polymorphic_on': type, > 'polymorphic_identity': 'employee' > } > > > > class Manager(Employee): > __mapper_args__ = { > 'polymorphic_identity': 'manager' > } > > > > class Engineer(Employee): > __mapper_args__ = { > 'polymorphic_identity': 'engineer' > } > > > > class EmployeeInTraversal(Employee): > """ > Wraps the Employee class. This is to keep our models and our > application > logic decoupled. > """ > def __getitem__(self, key): > """ > Make the employee behave like a traversal node. > > :param key: > The traversal key. If asked for tasks return the appropriate > root > factory > """ > if key == 'tasks': > return 'TasksRootFactory()' > raise KeyError > > > # Create the tables > Base.metadata.create_all(engine) > > > # Create both a Manager and an Engineer > manager = Manager(name='Taylor') > engineer = Engineer(name='Sam') > > > session = Session() > session.add_all([manager, engineer]) > session.commit() > > > # somewhere a request is made for /employee/1/tasks... > > > # Query for the engineer, somewhere in the EmployeeRootFactory > wrapped_engineer = session.query(EmployeeInTraversal).get(1) > > > # Traversal doing its thing. Here lies the trouble. > engineer_tasks = wrapped_engineer['tasks'] > > The last line of this code throws a TypeError with the message *'NoneType' > object has no attribute '__getitem__'*. Nothing is turned up by the query > to EmployeeInTraversal. If the query is instead made with the Employee > class, we get a similar TypeError: *'Manager' object has no attribute > '__getitem__'*. So in other words, querying EmployeeInTraversal returns > nothing, but querying Employee returns the object, although unwrapped. > > The reason for this, near as I can tell, is that the EmployeeInTraversal > class is being interpreted as a subtype of Employee rather than as a > wrapper for the class. I attribute this to the polymorphic nature of the > Employee class, but at this point I'm really just banging my head against > the wall. We would very much like to keep our models and application logic > separate, and not embed traversal logic in the schema classes... Is there > any way to wrap a polymorphic class without the wrapper being interpreted > as a sub-type? > > -- > You received this message because you are subscribed to the Google Groups > "sqlalchemy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sqlalchemy+...@googlegroups.com <javascript:>. > To post to this group, send email to sqlal...@googlegroups.com > <javascript:>. > Visit this group at http://groups.google.com/group/sqlalchemy. > For more options, visit https://groups.google.com/d/optout. > > >
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