SQLAlchemy：如何将混合属性设为声明属性？

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 。唯一的区别是每个时间戳都会减去不同的时间戳。

当然，使用declarative_attr不是必需的，但我需要属性是hybrid_property。

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

我还处于 0.6，所以我无法尝试你的奇特特性。 :)

但

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

您可以尝试以下操作：

class Duration(Base):

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

并将您的 *create_duration_field* 类方法移出该类。这样你也可以在不同的类上重用它。也许 declared_attr 也应该移到 *create_duration_field* 内，您必须在那里进行实验。

一个例子是我一直使用的这个小东西：

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

你会像这样使用它：

class SomeModel(Base):

    SomeInstance = constructor(primary_key=17)

instance = SomeModel.SomeInstance()

希望你明白了。

Answer 2

我已经运行了脚本并且没问题（0.9.6）。您只需要将代码从： 更改

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

为：

@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'>