Why not just wrap the python property:
class Obj1(Base):
__tablename__ = 'obj1'
id = Column(Integer, primary_key=True)
key = Column(String(20))
_value_rel = relationship("Obj2", uselist=False, backref="obj1")
@property
def value(self):
return self._value_rel and self._value_rel.value
@value.setter
def value(self, value):
if value is None:
self._value_rel = None
elif self._value_rel is None:
self._value_rel = Obj2(value=value)
else:
self._value_rel.value = value