Malcolm Tredinnick wrote: > > On Thu, 2008-09-11 at 15:36 +0800, Eric Abrahamsen wrote: >> I'm fooling around with comparing instances of different models, >> trying to make a unified stream of instances that can be sorted by >> date according to a certain datetime attribute on each model. I >> thought I'd define a __cmp__ method on each model in question, that >> would provide the appropriate attribute, rather than writing a sort >> key function. I started off just comparing instances of different >> models, to see what would happen, and found that some instances >> evaluate to 'greater than' other instances, but I don't know what the >> criteria is. It's not foreign key, and looking at the source I don't >> see anything defined for Model except for __eq__ and __ne__. Can >> someone enlighten? > > The "object" type/class has a __cmp__ method, essentially. It's > comparing id() of the instances. Having some kind of natural ordering on > models isn't something Django can do by default, since it's highly > domain dependent. Also, we need to define __eq__ on models so that > models with the same primary key of the same type hash as equal. That > means you can use model instances as dictionary keys, for example. > > Defining your own __lt__ and __gt__ methods is going to mess with that a > bit unless you're careful, so probably isn't recommend (it's going to be > confusing if "not a<b and not b<a" doesn't mean "a==b"). > A further consideration is that if you want to use the python sort() method on lists of model instances, the cmp() function will be called with many pairs of such instances. You will almost certainly find it quicker to define a key() function and provide that; the function is called once per model instance to create sort keys, which can be compared from within the C implementation of the sort() method without the need to call back out into Python code.
I've got some timings lying around somewhere that clearly show the decorate/sort/undecorate implementation is way faster under normal circumstances. regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-users@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/django-users?hl=en -~----------~----~----~----~------~----~------~--~---
