the original mapping essentially expressed this relationship:

A <-- cascade="all, delete-orphan" --> B

in both directions. What that means is, there cannot be an A with no B in the database, and there cannot be a B with no A in the database, i.e. its either A<->B or both will be deleted. its like an oxygen molecule, or something.

what you need to do is decide which of A and B can exist on its own without a parent relationship. since you want to delete rows from B and not A, that would indicate that the mapping should be:

mapper(A, a) mapper( B, b, properties={ 'a': relation( A, backref=backref('b', lazy=False, uselist=False, cascade='all, delete-orphan'), uselist=False ) } )

i.e. the "delete-orphan" cascade is only in the direction from A->B. the cascade from B->A is left at its default value of "save-update", so delete operations dont propigate from B's to A's.