How to write a common get_by_id() method for all kinds of models in Sqlalchemy?

Question

I'm using pylons with sqlalchemy. I have several models, and found myself wrote such code again and again:

question = Session.query(Question).filter_by(id=question_id).one()
answer = Session.query(Answer).fileter_by(id=answer_id).one()
...
user = Session.query(User).filter_by(id=user_id).one()

Since the models are all extend class Base, is there any way to define a common get_by_id() method?

So I can use it as:

quesiton = Question.get_by_id(question_id)
answer = Answer.get_by_id(answer_id)
...
user = User.get_by_id(user_id)

Answer 1

class Base(object):
    @classmethod
    def get_by_id(cls, session, id):
        q = session.query(cls).filter_by(id=id)
        return q.one()

Question.get_by_id(Session, question_id)

Answer 2

Unfortunately, SQLAlchemy doesn't allow you to subclass Base without a corresponding table declaration. You could define a mixin class with get_by_id as a classmethod, but then you'd need to specify it for each class.

A quicker-and-dirtier solution is to just monkey-patch it into Base:

def get_by_id(cls, id, session=session):
    return session.query(cls).filter_by(id=id).one()

Base.get_by_id = classmethod(get_by_id)

This assumes you've got a session object available at definition-time, otherwise you'll need to pass it as an argument each time.

Answer 3

If id is your primary key column, you just do:

session.query(Foo).get(id)

which has the advantage of not querying the database if that instance is already in the session.