Question

是否可以在SQLAlchemy中强制分配给映射列的最大字符串长度值？如果指定的字符串值比STRING类型的相应表列的长度长，那么我想要的是引发异常。

谢谢

Answer 1

最简单的方法是重命名映射列并通过属性代理它：

class Something(Base):
    ...
    _foo = Column('foo', String(123))

    @property
    def foo(self):
        return self._foo

    @foo.setter
    def foo(self, value):
        if len(value) > _foo.type.length:
            raise Exception("Value too long")
        self._foo = value

您可以轻松地分解属性创建，甚至使用像formencode这样的通用验证框架。

如果您需要更多SQLAlchemy特定解决方案并且不介意使用特定接口，则SQLAlchemy具有用于捕获属性上的事件的扩展机制。使用它的验证器看起来像这样：

from sqlalchemy.orm.interfaces import AttributeExtension, InstrumentationManager
from sqlalchemy.orm import ColumnProperty

class InstallValidatorListeners(InstrumentationManager):
    def post_configure_attribute(self, class_, key, inst):
        """Add validators for any attributes that can be validated."""
        prop = inst.prop
        # Only interested in simple columns, not relations
        if isinstance(prop, ColumnProperty) and len(prop.columns) == 1:
            col = prop.columns[0]
            # if we have string column with a length, install a length validator
            if isinstance(col.type, String) and col.type.length:
                inst.impl.extensions.insert(0, LengthValidator(col.type.length))

class ValidationError(Exception):
    pass

class LengthValidator(AttributeExtension):
    def __init__(self, max_length):
        self.max_length = max_length

    def set(self, state, value, oldvalue, initiator):
        if len(value) > self.max_length:
            raise ValidationError("Length %d exceeds allowed %d" %
                                (len(value), self.max_length))
        return value

然后，您可以通过在要验证的任何课程上设置__sa_instrumentation_manager__ = InstallValidatorListeners来使用此扩展程序。如果希望它应用于从它派生的所有类，也可以在Base类上设置它。

Answer 2

以下版本适用于较新的sqlalchemy版本的事件系统：

class InstallValidatorListeners(InstrumentationManager):
    def post_configure_attribute(self, class_, key, inst):
        """Add validators for any attributes that can be validated."""
        prop = inst.prop
        # Only interested in simple columns, not relations
        if isinstance(prop, ColumnProperty) and len(prop.columns) == 1:
            col = prop.columns[0]
            if isinstance(col.type, String) and col.type.length:
                sqlalchemy.event.listen(
                    getattr(class_, key), 'set', LengthValidator(col.type.length), retval=True)


class ValidationError(Exception):
    pass


class LengthValidator(AttributeExtension):
    def __init__(self, max_length):
        self.max_length = max_length

    def __call__(self, state, value, oldvalue, initiator):
        if len(value) > self.max_length:
            raise ValidationError(
                "Length %d exceeds allowed %d" % (len(value), self.max_length))
        return value

SQLAlchemy - 最大列长度

2 个答案: