SQLAlchemy:如何让hybrid_property成为declared_attr？

Question

我希望能够使用不同的参数调用Duration.create_duration_field()，并在我的类上创建多个hybrid_property。唯一的区别是它们每个都会减去不同的时间戳。

当然，使用declarative_attr不是必需的，但我需要这些属性是hybrid_propertys。

import datetime
from sqlalchemy import create_engine, MetaData
from sqlalchemy import Column, String, DateTime, Integer
from sqlalchemy.ext.declarative import declarative_base, declared_attr
from sqlalchemy.ext.hybrid import hybrid_property
from sqlalchemy.orm import sessionmaker


metadata = MetaData()
Base = declarative_base(metadata=metadata)


class Duration(Base):
    __tablename__ = "duration"

    pk = Column(Integer, primary_key=True)
    name = Column(String, nullable=False)
    notes = Column(String)
    timestamp_initiated = Column(DateTime,
                                default=datetime.datetime.now(),
                                nullable=False)
    timestamp_done = Column(DateTime)

    def __unicode__(self):
        return self.name

    @declared_attr
    def duration(cls):
        return cls.create_duration_field("initiated", "done")

    @classmethod
    def create_duration_field(cls, start, end):
        @hybrid_property
        def duration(obj):
            getattr(obj, "timestamp_%s" % end) - getattr(obj, "timestamp_%s" % start)
        @duration.expression
        def duration(cls):
            return getattr(cls, "timestamp_%s" % end) - getattr(cls, "timestamp_%s" % start)
        return duration



engine = create_engine('sqlite:///:memory:', echo=True)
metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()

duration = Duration(name="Test", timestamp_done=datetime.datetime.now() + datetime.timedelta(seconds=25))
session.add(duration)
session.commit()

assert isinstance(duration.duration, datetime.timedelta)

目前，duration.duration是对hybrid_property的引用

<sqlalchemy.ext.hybrid.hybrid_property object at 0x1d02190>

Answer 1

我仍然是0.6，所以我不能尝试你的花哨属性。:)

但它不是

class Duration(Base):
    # [...] 
    @declared_attr
    def duration(cls):
        return cls.create_duration_field("initiated", "done")

您可以尝试如下所示：

class Duration(Base):

    duration = declared_attr(create_duration_field("initiated", "done"))

并将您的*create_duration_field*类方法从该类中移出。这样，您还可以在不同的类上重用它。也许declared_attr也应该迁移到*create_duration_field*中，你必须在那里进行实验。

我经常使用的这个小东西就是一个例子：

def constructor(**kw):                                                          
    """Creates a class constructor class method"""
    def load(cls):
        query = cls.query
        return query.filter_by(**kw).one()
    return classmethod(load)

您可以这样使用它：

class SomeModel(Base):

    SomeInstance = constructor(primary_key=17)

instance = SomeModel.SomeInstance()

希望你能明白我的意思。

Answer 2

我已经运行了脚本，它是正常的(0.9.6)。您只需将代码更改为：

@hybrid_property
def duration(obj):
    getattr(obj, "timestamp_%s" % end) - getattr(obj, "timestamp_%s" % start)

至：

@hybrid_property
def duration(obj):
    return getattr(obj, "timestamp_%s" % end) - getattr(obj, "timestamp_%s" % start)

结果：

duration.duration = 0:00:25.017861 datetime.timedelta(0, 25, 17861) <type 'datetime.timedelta'>