I have a base class which has two subclasses specified via a polymorphic
discriminator, and I'm wondering how to set things up so the returned
results are instances of the subclass, when I create an instance of the
base class and assign the relevant value to the discriminator column.
Here's my example code:
import sys
from sqlalchemy import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm.attributes import instance_dict
from sqlalchemy.orm import relation, backref, class_mapper, create_session
e = create_engine('sqlite:////tmp/foo.db', echo=True)
Base = declarative_base(bind=e)
class Test(Base):
__tablename__ = 'test'
id = Column(Integer, primary_key=True)
type = Column(String(12))
name = Column(String(50))
__mapper_args__ = {
'polymorphic_identity': 'base',
'polymorphic_on': type,
'with_polymorphic': '*',
}
class TestOne(Test):
__tablename__ = 'test1'
id = Column(Integer, ForeignKey('test.id'), primary_key=True)
value1 = Column(String(16))
__mapper_args__ = {
'polymorphic_identity': 'one',
}
class TestTwo(Test):
__tablename__ = 'test2'
id = Column(Integer, ForeignKey('test.id'), primary_key=True)
value2 = Column(String(16))
__mapper_args__ = {
'polymorphic_identity': 'two',
}
if __name__ == '__main__':
Base.metadata.drop_all()
Base.metadata.create_all()
session = create_session(bind=e, autocommit=False)
test1 = Test(type='one', name='a test')
session.add(test1)
print test1
This returns an instance of Test, not Test1, despite the discriminator
indicating that it should be cast as Test1.
Is it possible to handle this in a way which will yield an instance of
Test1?
Also, if i create test1, and then do session.expunge_all, then immediately
requery for session.query(Site).get(1), the returned instance is of type
Test1, but trying to set the value for value1 yields 'Can't update table
using NULL for primary key value'. Is there a way to fix this, so the
necessary table row is created with the same primary key, instead of
leaving it blank? It seems that all the info needed to handle the
auto-creation of the sub-table is present, but it just fails.
Thanks
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