SQLAlchemy中Relationship的validates赋值时不触发，如何解决？

一、让直接赋值也触发验证 #

@validates('failure_modes')仅监听集合内元素的增减操作（比如append），直接赋值整个列表属于替换属性值，不会触发该验证。要覆盖这种场景，需使用SQLAlchemy的属性事件监听set动作：

from sqlalchemy.orm import validates, Mapped, mapped_column, relationship
from sqlalchemy import event
from datetime import datetime
from typing import List

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):
        """验证单个failure_mode添加时的状态"""
        self.passed = all(fm.passed for fm in self.failure_modes + [failure_mode])
        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):
        if self.test:
            self.test.passed = all(fm.passed for fm in self.test.failure_modes)
        return passed

# 监听failure_modes的赋值事件
@event.listens_for(DeviceTestResult.failure_modes, 'set')
def on_failure_modes_set(target, new_value, old_value, initiator):
    # 重新计算passed状态
    target.passed = all(fm.passed for fm in new_value)
    return new_value

说明：

• event.listens_for会在failure_modes被直接赋值时触发，这里直接基于新列表的所有元素重新计算passed状态，逻辑更简洁。
• 原有的validate_failure_modes仍处理元素增减的场景，两者配合覆盖所有修改方式。

二、禁止直接赋值，仅允许append/extend修改 #

如果要强制用户只能通过append/extend等方法修改集合，可以自定义受限列表类，替换默认的集合类型：

class RestrictedList(list):
    def __setitem__(self, key, value):
        """禁止通过索引赋值"""
        raise AttributeError("Cannot assign via index. Use append/extend instead.")
    
    def __setattr__(self, name, value):
        """禁止直接修改属性（避免绕过限制）"""
        if name not in ('__dict__', '__len__'):
            raise AttributeError("Cannot assign directly to this list. Use append/extend instead.")
        super().__setattr__(name, value)

class DeviceTestResult:
    __tablename__ = "device_test"

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

    # 其余代码保持不变...

说明：

• default_factory确保初始化时创建RestrictedList实例，collection_class指定SQLAlchemy加载关联数据时使用该类。
• 当用户尝试device.failure_modes = [failure_mode]或device.failure_modes[0] = xxx时，会抛出AttributeError，强制使用集合的添加方法。

内容的提问来源于stack exchange，提问作者frimann