> -----Original Message----- > From: sqlalchemy@googlegroups.com > [mailto:sqlalch...@googlegroups.com] On Behalf Of Crusty > Sent: 23 September 2009 15:48 > To: sqlalchemy@googlegroups.com > Subject: [sqlalchemy] unexpected chained relations and > "append" behaviour > > > Hello everyone, > > I have a realy simple model for you to consider: > > 1 car has n wheels > car.wheels is a relation from cars to wheels > wheel.car is a backref to cars > > 1 car has n parts > car.parts is a relation from car to parts > > I just wondered why my app was really getting slow, turned on SA debug > mode, and saw that > > my_new_doorknob = model.Part("doorknob") > wheel.car.parts.append(my_new_door_knob) > > is downloading the entire "parts" table WHERE parts.car == car.id > (that is around 20.000 entries) just so that it can append my new > doorknob to that relation. > > Furthermore I noticed a similar behaviour when doing > something like this: > > amount_of_parts = len(car.parts) > > Instead of sending a COUNT to the database, it populates the entire > car.parts relation (around 20.000 entries) just to get the count. Of > course I could avoid using relations, and just use my __init__ > functions, or setting: > > my_new_doorknob = model.Part("doorknob") > my_new_doorknob.car_id = car.id > DBSession.append(my_new_doorknob) > > But then I could as well just write literal SQL if I cant use the "R" > part of ORM... > > Has anyone observed similar behaviour or is this a "feature" and > intended to work like this? > > Greetings, Tom
Yes, this is exactly how it is intended to work. You may like to read http://www.sqlalchemy.org/docs/05/mappers.html#working-with-large-collec tions for hints on how to improve performance. In particular, making your car.parts property a 'dynamic' relation rather than the default will prevent SA from loading the entire collection unless you specifically ask it to. However, the len(car.parts) line won't work. SA deliberately doesn't implement the __len__ method for Query objects because it is called implicitly by python in a number of situations, and running a potentially slow query when you aren't expecting it is a bad idea. Instead you would use car.parts.count(). Hope that helps, Simon --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To post to this group, send email to sqlalchemy@googlegroups.com To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en -~----------~----~----~----~------~----~------~--~---
