Replicate the example from https://docs.sqlalchemy.org/en/14/_modules/examples/inheritance/joined.html using classic mappings

classic_mapping.py

from pathlib import Path

import attr
from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import MetaData
from sqlalchemy import String
from sqlalchemy import Table
from sqlalchemy.orm import registry
from sqlalchemy.orm import relationship
from sqlalchemy.orm import sessionmaker


class Person:
    pass


@attr.s(auto_attribs=True)
class Manager(Person):
    name: str
    data: str


@attr.s(auto_attribs=True)
class Engineer(Person):
    name: str
    info: int


@attr.s(auto_attribs=True)
class Company:
    people: list[Person]


metadata = MetaData()
mapper_registry = registry()

persons_table = Table(
    "person",
    metadata,
    Column("id", Integer, primary_key=True, autoincrement=True),
    Column("type", String(50)),
)

managers_table = Table(
    "manager",
    metadata,
    Column('id', ForeignKey("person.id"), primary_key=True),
    Column("name", String(50)),
    Column("data", String(50)),
)

engineers_table = Table(
    "engineer",
    metadata,
    Column('id', ForeignKey("person.id"), primary_key=True),
    Column("name", String(50)),
    Column("info", Integer),
)


company_table = Table(
    "company",
    metadata,
    Column("id", Integer, primary_key=True),
)

company_2_people_table = Table(
    "company_2_people",
    metadata,
    Column("id", Integer, primary_key=True, autoincrement=True),
    Column("company_id", ForeignKey("company.id")),
    Column("person_id", ForeignKey("person.id")),
)

person_mapper = mapper_registry.map_imperatively(
    Person,
    persons_table,
    polymorphic_identity="person",
    polymorphic_on=persons_table.c.type,
)
manager_mapper = mapper_registry.map_imperatively(
    Manager,
    managers_table,
    inherits=person_mapper,
    polymorphic_identity="manager",
)
engineer_mapper = mapper_registry.map_imperatively(
    Engineer,
    engineers_table,
    inherits=person_mapper,
    polymorphic_identity="engineer",
)

company_mapper = mapper_registry.map_imperatively(
    Company,
    company_table,
    properties={
        "people": relationship(
            person_mapper,
            secondary=company_2_people_table,
            collection_class=list,
        ),
    },
)

fn = Path(__file__).with_suffix(".db")
fn.unlink(missing_ok=True)
engine = create_engine(f"sqlite:///{fn}", echo=False)
metadata.create_all(engine)

Session = sessionmaker(bind=engine)
with Session() as session:
    m1 = Manager(name="Manager 1", data="Manager Data")
    m2 = Manager(name="Manager 2", data="Manager Data")
    e1 = Engineer(name="Eng 1", info=10)
    e2 = Engineer(name="Eng 2", info=30)

    company = Company([e2, m1, e1, m2])
    session.add(company)
    session.commit()

with Session() as session:
    print(session.query(Company).get(1))

Asked for help on Stack Overflow: https://stackoverflow.com/questions/66921914/using-polymorphic-classes-in-classic-mappings-style

Found the solution, the code above now works as expected. 👍