allow_unmapped seems to not be honored in all cases #9369

from typing import List
from sqlalchemy import Column, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.types import Integer
from sqlalchemy.orm import declarative_base


# qualify the base with __allow_unmapped__.  Can also be
# applied to classes directly if preferred
class Base:
    __allow_unmapped__ = True

    # raises, but shouldn't (right?)
    id: int = Column(Integer, primary_key=True)


Base = declarative_base(cls=Base)


# existing mapping proceeds, Declarative will ignore any annotations
# which don't include ``Mapped[]``
class Foo(Base):
    __tablename__ = "foo"

    bars: List["Bar"] = relationship("Bar", back_populates="foo")


class Bar(Base):
    __tablename__ = "bar"
    foo_id = Column(ForeignKey("foo.id"))

    foo: Foo = relationship(Foo, back_populates="bars", cascade="all")