I am building a polymorphic relationship between classes A and B through a Link class.
I want to achieve that when an A object is deleted, the linked B objects are deleted too.
To do that, I defined a relationship between A and Link, with a cascade relationship all, delete, in this way as soon as A is deleted we will try to delete all the links, and then I intercepted the delete on the Link using an event listener before_delete class and I used it to delete also the B object.
A better solution can exist, and most of all I am concerned about the connection object in the event listener that doesn’t seem to be bound to any session.

class A(BaseORMModel):
   id: Mapped[int] = mapped_column(
        primary_key=True,
    )
   links = relationship(
        "Link",
        primaryjoin="and_("
        "foreign(Link.source_id)==A.id,"
        "Link.source_type=='TypeA'"
        ")",
        cascade="all, delete",
    )

class B(BaseORMModel):
   id: Mapped[int] = mapped_column(
        primary_key=True,
    )

class Link(BaseOrmModel):
id: Mapped[int] = mapped_column(
        primary_key=True,
    )

    source_id: Mapped[int] = mapped_column(
        index=True,
    )

    source_type: Mapped[str]

    related_id: Mapped[int] = mapped_column(
        index=True,
    )

    related_type: Mapped[str]

@event.listens_for(Link, "before_delete")
def delete_links(mapper, connection, Link):
    if Link.related_type == 'TypeB':
        q = delete(B).where(B.id == Link.related_id)
        connection.execute(
            q
        )
        print(q.compile(compile_kwargs={"literal_binds": True}))