its likely a bug. there are some boolean tests for
"polymorphic_identity" in mapper.py which should be changed to "is
None".
On Jan 30, 2009, at 11:58 AM, Gunnlaugur Thor Briem wrote:
> Hi,
>
> [trying to send this again, seems like previous copy got lost in
> some moderation queue]
>
> Messing with single-table inheritance in a declarative model, with a
> non-abstract base class, I find that querying fails if
> polymorphic_identity is 0 (zero). Example:
>
> ################ code begins ################
> from sqlalchemy import Column, Integer, create_engine
> from sqlalchemy.ext.declarative import declarative_base
> from sqlalchemy.orm import sessionmaker
>
> Base = declarative_base()
> class Course(Base):
> __tablename__ = 'course'
> id = Column(Integer, primary_key=True)
> course_type = Column(Integer)
> __mapper_args__ = {'polymorphic_on':course_type,
> 'polymorphic_identity':0}
>
> class MultiYearCourse(Course):
> __mapper_args__ = {'polymorphic_identity':1}
>
> engine = create_engine('sqlite:///:
> memory:')
> Base.metadata.create_all(bind=engine)
> session = sessionmaker(bind=engine)()
>
> myc = MultiYearCourse()
> myc.name = u"Computer Graphics"
> c = Course()
> c.name = u"Sociology"
> session.add(c)
> session.add(myc)
> session.commit()
>
> print "MYC: %s" % myc
> print "C: %s" % c
> query = session.query(Course)
> print "Query: %s" % query
> print "Results: %s" % query.all()
> ################ code ends ################
>
> That last line fails with an AssertionError:
>
> ################ output begins ################
> MYC: <__main__.MultiYearCourse object at 0xcf7d30>
> C: <__main__.Course object at 0xcf7d70>
> Query: SELECT course.id AS course_id, course.course_type AS
> course_course_type
> FROM course
> Traceback (most recent call last):
> File "/Users/gthb/Documents/workspace/test/src/sqlalchemytest.py",
> line 31, in <module>
> print "Results: %s" % query.all()
> File "/Users/gthb/Library/Python/2.5/site-packages/
> SQLAlchemy-0.5.2-py2.5.egg/sqlalchemy/orm/query.py", line 1186, in all
> return list(self)
> File "/Users/gthb/Library/Python/2.5/site-packages/
> SQLAlchemy-0.5.2-py2.5.egg/sqlalchemy/orm/query.py", line 1341, in
> instances
> rows = [process[0](context, row) for row in fetch]
> File "/Users/gthb/Library/Python/2.5/site-packages/
> SQLAlchemy-0.5.2-py2.5.egg/sqlalchemy/orm/query.py", line 1942, in
> main
> return _instance(row, None)
> File "/Users/gthb/Library/Python/2.5/site-packages/
> SQLAlchemy-0.5.2-py2.5.egg/sqlalchemy/orm/mapper.py", line 1557, in
> _instance
> _instance = polymorphic_instances[discriminator]
> File "/Users/gthb/Library/Python/2.5/site-packages/
> SQLAlchemy-0.5.2-py2.5.egg/sqlalchemy/util.py", line 71, in
> __missing__
> self[key] = val = self.creator(key)
> File "/Users/gthb/Library/Python/2.5/site-packages/
> SQLAlchemy-0.5.2-py2.5.egg/sqlalchemy/orm/mapper.py", line 1695, in
> configure_subclass_mapper
> raise AssertionError("No such polymorphic_identity %r is
> defined" % discriminator)
> AssertionError: No such polymorphic_identity 0 is defined
> ################ output ends ################
>
> But if I exchange the polymorphic identities, so the base class gets
> the 1 and the subclass gets the 0, then it runs just fine!
>
> It seems to me that this can't be intentional — don't see a reason
> for it, and the docs do not mention any particular restrictions on
> values of polymorphic_identity.
>
> Regards,
>
> - Gulli
>
> >
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