Consider the following using sqlalchemy:

from sqlalchemy.orm import validates

class DeviceTestResult:
    __tablename__ = "device_test"

    passed: bool = mapped_column(default=False, init=False)
    failure_modes: Mapped[list['FailureMode']] = relationship(default_factory=list, back_populates='test')

    @validates('failure_modes')
    def validate_failure_modes(self, key, failure_mode):
        """
        Validator for failure modes. When attribute is changed, updates passed to be false if there are any fails.
        """
        currently_passes = all([fm.passed for fm in self.failure_modes])
        self.passed = all([fm for fm in [currently_passes, failure_mode.passed]])
        return failure_mode


class FailureMode:
    __tablename__ = "failure_mode"
    
    passed: bool = mapped_column()
    test: Mapped['DeviceTestResult'] = relationship(back_populates='failure_modes', default=None)
    completed_at: datetime = mapped_column(insert_default=datetime.now(), default=datetime.now())

    @validates('passed')
    def validate_test(self, key, passed):
        """
        validator for passed. Updates associated test object status
        """
        if self.test:
            currently_passes = all([fm.passed for fm in self.test.failure_modes])
            self.test._passed = all([fm for fm in [currently_passes, passed]])
        return passed

# Case 1 - append, emits validate_failure_modes
device_test = DeviceTest()
failure_mode = FailureMode()
device.failure_modes.append(failure_mode)

# Case 2 - assignment, does not emit validate_failure_modes
device.failure_modes = [failure_mode]

validate_failure_modes is emitted in case 1, but not in case 2.

How can I get validation in case 2? If that’s not possible, how can I restrict a user from setting the attribute using assignment, so that they can only modify via appending?

I’ve experimented with using sqlalchemy descriptors & hybrids but haven’t gotten it to work.