SQLAlchemy: How to make a hybrid_property a declared_attr?

Question

I would like to be able to call Duration.create_duration_field() with different parameters and have more than one hybrid_property created on my class. The only difference would be that different timestamps will be subtracted for each of them.

Of course using declarative_attr is not a requirement, but I need the properties to be hybrid_property s.

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)

Currently duration.duration is a reference to hybrid_property

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

Answer 1

I've run the script and it's ok (0.9.6). You just need to change the code from:

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

to:

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

Result:

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

Answer 2

I'm still at 0.6 so I can't try your fancy properties. :)

But instead of

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

you could try something like:

class Duration(Base):

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

and move your *create_duration_field* classmethod off that class. That way you can also reuse it on different classes. Perhaps declared_attr should also be moved inside *create_duration_field*, you'll have to experiment there.

An example would be this little thing I use all the time:

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

You would use it like this:

class SomeModel(Base):

    SomeInstance = constructor(primary_key=17)

instance = SomeModel.SomeInstance()

Hope you get the idea.