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.
>
> 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
>
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
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> To post example code, please provide an MCVE: Minimal, Complete, and
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SQLAlchemy -
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http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
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