一次写入只读字段

Question

我们需要使用SQLAlchemy/Elixir实现一次写入(在对象创建时)只读字段.

一个快速而肮脏的解决方案:

class User(Entity):
    # some fields ...    
    _created    = Field(DateTime, default=datetime.utcnow)
    @property
    def created(self):
        return self._created

有没有更好的方法(例如,使用插入前触发器？)

Answer 1

可能有很多方法,但一个是使用@validates:

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class A(Base):
    __tablename__ = 'a'

    id = Column(Integer, primary_key=True)

    readonly1 = Column(String)
    readonly2 = Column(String)


    @validates('readonly1', 'readonly2')
    def _write_once(self, key, value):
        existing = getattr(self, key)
        if existing is not None:
            raise ValueError("Field '%s' is write-once" % key)
        return value


a1 = A()
a1.readonly1 = 'foo'
assert a1.readonly1 == 'foo'

try:
    a1.readonly1 = 'bar'
    assert False
except ValueError, e:
    print e

e = create_engine("sqlite://")
Base.metadata.create_all(e)
s = Session(e)
s.add(A(readonly1='foo', readonly2='bar'))
s.commit()

a2 = s.query(A).first()

try:
    a2.readonly2 = 'bar2'
    assert False
except ValueError, e:
    print e

@validates只是使用属性事件的简写,您可以使用它来构建其他设置方法.