[Sqlalchemy-tickets] [sqlalchemy] #2750: Inconsistent behavior for polymorphic queries

[sqlalchemy <mi...@zz...>]

#2750: Inconsistent behavior for polymorphic queries
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 Reporter:  schlamar  |           Owner:  zzzeek
     Type:  defect    |          Status:  new
 Priority:  medium    |       Milestone:
Component:  orm       |        Severity:  no triage selected yet
 Keywords:            |  Progress State:  awaiting triage
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 This is my test case:


 {{{
 #!python

 from sqlalchemy import create_engine, Column, ForeignKey, Integer, String
 from sqlalchemy.orm import sessionmaker
 from sqlalchemy.ext.declarative import declarative_base

 Base = declarative_base()


 class Model(Base):
     __tablename__ = 'model'
     id = Column(Integer, primary_key=True)
     label = Column(String)

     mtype = Column(Integer, nullable=False)
     __mapper_args__ = {'polymorphic_on': mtype}


 class SubModel(Model):
     __tablename__ = 'submodel'
     id = Column(Integer, ForeignKey('model.id'), primary_key=True)
     __mapper_args__ = {'polymorphic_identity': 1}


 def main():
     engine = create_engine('sqlite:///:memory:')
     Base.metadata.create_all(engine)
     Session = sessionmaker(bind=engine)
     session = Session()

     m = Model(mtype=1)
     m.label = 'x'
     session.add(m)
     session.commit()
     m_id = m.id

     session = Session()
     m = session.query(Model).get(m_id)
     m.label = 'y'
     session.commit()
     print m.id
 }}}

 This yields the following error:


 {{{
 Traceback (most recent call last):
   File "sqla_test.py", line 44, in <module>
     main()
   File "sqla_test.py", line 40, in main
     print m.id
   File "D:\Projekte\HACS\packages\sqlalchemy\orm\attributes.py", line 316,
 in __get__
     return self.impl.get(instance_state(instance), dict_)
   File "D:\Projekte\HACS\packages\sqlalchemy\orm\attributes.py", line 611,
 in get
     value = callable_(passive)
   File "D:\Projekte\HACS\packages\sqlalchemy\orm\state.py", line 375, in
 __call__
     self.manager.deferred_scalar_loader(self, toload)
   File "D:\Projekte\HACS\packages\sqlalchemy\orm\loading.py", line 606, in
 load_scalar_attributes
     raise orm_exc.ObjectDeletedError(state)
 sqlalchemy.orm.exc.ObjectDeletedError: Instance '<SubModel at 0x2f0eeb0>'
 has been deleted, or its row is otherwise not
 present.
 }}}

 I have already investigated and can explain the behavior: the query
 returns actually a !SubModel instance because of `mtype == 1`. After the
 commit the object is expired so a new query is issued which joins model
 against submodel. But an entry in the submodel table was never created.

 This behavior is really inconsistent. SQLAlchemy should throw an error on
 the query or on the insert. Alternatively, it might fix the insert
 automatically by creating an corresponding entry to submodel.

 Besides, if you issue `session.query(SubModel).get(m_id)` you get the same
 error message, which is really misleading.

--
Ticket URL: <http://www.sqlalchemy.org/trac/ticket/2750>
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