Unable to query db inside factory ('Iterator' object has no attribute '_sa_instance_state')

I got this factory:

class DepartmentFactory(factory.alchemy.SQLAlchemyModelFactory):
    class Meta:
        model = models.Department
        sqlalchemy_session = Session

    abbreviation = 'MATH'

    @factory.lazy_attribute
    def school(self):
        return factory.Iterator(Session.query(models.School).all())

And I get this error: AttributeError: 'Iterator' object has no attribute '_sa_instance_state'

I’ve checked and made sure that the query returns a list of School objects.

[fgblomqvist]

Turning it into a generator function fixed it, thanks for the extensive explanation. Will use that for now. Treating it differently (and wrapping it accordingly) depending on if it’s an SQLAlchemy one or not feels the most natural to me (aka. the user does not have to care), but that is surely your decision to make.

Thanks for the in-depth explanation about the lack of self, the more you know! 😄 It was a bit of an eye-sore both for me and my IDE so solved it this way:

def schools():
    yield from models.School.query.all()


class DepartmentFactory(factory.alchemy.SQLAlchemyModelFactory):
    class Meta:
        model = models.Department
        sqlalchemy_session = models.db.session

    abbreviation = factory.Iterator(['COEN', 'MATH', 'POLI', 'MECH'])
    name = 'Sample Department'
    school = factory.iterator(schools)

😜

[rbarrois]

Oh, thanks for the stack!

Indeed, the factory.Iterator / factory.iterator functions work best with a generator function. It should work with:

@factory.iterator
def school():
    yield from Session.query(models.School).all()

Let’s dive under the hood here: a generator function (anything containing yield) returns a special object, which will only execute the function’s code (up to the next yield statement) once next(x) is called on it.

Django’s querysets are built upon generators, and work natively; however, it seems that SQLAlchemy’s querysets will evaluate directly; using a yield from x is equivalent to for i in x: yield i, and “wraps” the call in a generator.

This could be fixed either through better docs, or in the code; I’ll have to see which way feels most natural.

Regarding the lack of self:

There is some “dark magic” occurring in factory_boy: basically, the engine will collect all attributes from your class definition, and store them in some internal structures for later use.

I use the self parameter to calls because it feels more natural, but that’s actually a trick 😉 When creating an object, a method of those definitions is called with a stub for the attributes of the object to be built. Once that stub is filled, its attributes are used as kwargs for the actual model.

Since an iterator cannot be tuned for the object being built (whereas a lazy_attribute is designed for that!), there is no self parameter.