Inheritance Hierarchies: Loading a model created with parent class in same session

[David Bücher]

To avoid writing a function selecting the correct class for a 'type' value, I would like to create models using the parent class ('Employee') and 'type' instead of using the child class ('Manager', 'Engineer') in the first place. I noticed that the resulting object is not of the respecting child class (e.g. 'Manager'), even after re-querying it with the child class or with with_polymorphic. However, only in a new session this works out. Is there a way to achieve this without opening a new session?

Here is my example script - I am using sqlalchemy version 1.4.37:

from sqlalchemy import Column, Integer, String, create_engine
from sqlalchemy.orm import declarative_base, \
    relationship, sessionmaker

engine = create_engine('sqlite:///:memory:', echo=True)
Base = declarative_base()

class Employee(Base):
    __tablename__ = "employee"
    id = Column(Integer, primary_key=True)
    name = Column(String(50))
    type = Column(String(50))

    __mapper_args__ = {
        "polymorphic_identity": "employee",
        "polymorphic_on": type,
        'with_polymorphic': '*'
    }

class Manager(Employee):
    manager_data = Column(String(50))

    __mapper_args__ = {
        "polymorphic_identity": "manager"
    }

Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()
manager = Employee(name='name of Manager', type='manager')
session.add(manager)
session.flush()
manager_id = manager.id
session.commit()

# doesn't work:
m0 = session.query(Employee).filter_by(id=manager_id).first()
print(">>", m0.__mapper_args__['polymorphic_identity'])
try:
    print(m0.manager_data)
except:
    print(">> didn't work")

# doesn't work:
m = session.query(Manager).filter_by(id=manager_id).first()
print(">>", m.__mapper_args__['polymorphic_identity'])
try:
    print(m.manager_data)
except:
    print(">> didn't work")

# works:
session.close()
session2 = Session()
m1 = session2.query(Employee).filter_by(id=manager_id).first()
print(">>", m1.__mapper_args__['polymorphic_identity'])
print(m1.manager_data)

# works:
m = session2.query(Manager).filter_by(id=manager_id).first()
print(">>", m.__mapper_args__['polymorphic_identity'])
print(m.manager_data)

session2.close()

Thanks,
David

[Mike Bayer]

On Sun, Jun 19, 2022, at 3:52 PM, David Bücher wrote:

To avoid writing a function selecting the correct class for a 'type' value, I would like to create models using the parent class ('Employee') and 'type' instead of using the child class ('Manager', 'Engineer') in the first place.

that pattern is not supported by SQLAlchemy's inheritance feature.

I noticed that the resulting object is not of the respecting child class (e.g. 'Manager'), even after re-querying it with the child class or with with_polymorphic. However, only in a new session this works out.

that's because the object you've created is already in the identity map (https://docs.sqlalchemy.org/en/14/glossary.html#term-identity-map) and is not re-constructed.

Is there a way to achieve this without opening a new session?

overall just simply dont use the inheritance feature and just have an Employee class with a type column.   it does not appear you seeking to make any use of the inheritance feature.

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