Hello Simon, thanks for your answer, I will have a look into that. By the way: len(car.parts) does indeed work, try it ;)
Greetings, Tom On Wed, Sep 23, 2009 at 4:57 PM, King Simon-NFHD78 <simon.k...@motorola.com> wrote: > >> -----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 -~----------~----~----~----~------~----~------~--~---
