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.t...@gmail.com> 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?
>
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