I read that post, and now have a general idea of what to do, but I'm still confused on the specifics. It doesn't help that my SQL is VERY rusty.
Am I supposed to use cursor.execute() to do the INITIAL lookup? That is, instead of calling a filter, or whatnot, on each book entry, I do a cursor.execute that gets the books I want AND the related living authors in one fell swoop? Do I set that up as a custom Manager? What would be the SQL for that? My model does not create an in between table (such as appname_book_authors). How do I append my results to a queryset? I tried to figure these things out on my own, but I just couldn't wrap my head around it. On Aug 26, 2:32 am, Eric Abrahamsen <[EMAIL PROTECTED]> wrote: > On Aug 26, 2008, at 5:00 PM,MrJogowrote: > > > > > I guess I saw it as operating on a group of objects: filtering the > > group of authors related to my_book by is_living. I also think I got > > RelatedManager confused with Manager. > > Yaar, it can be a bit confusing. I guess what's important is to > remember which model you've started your query with – all queries are > relative to that table. I wonder if it's possibly to subclass > RelatedManager and add custom methods, come to think of it... > > > > > I think I can handle two db hits, although it's not optimal. I wish > > there was a way to get a set of data filtered on many levels (ie, only > > books with living authors AND only the related living authors) with > > one command/db hit. > > It will be more than two db hits if you've got to call > living_authors() on every book in the set! > > Actually I needed to do something like this recently, here's a link to > the relevant > thread:http://groups.google.com/group/django-users/browse_thread/thread/9be1... > > There's a lot of cruft in that thread, the basic idea is using > cursor.execute() to do your subqueries (selecting all living authors > per book), and then using pure python to attach those subqueries to > your larger book queryset. It's not as painful as it sounds. > > Lastly, there are rumors of aggregation support coming soon/already > here, but I'm not sure how that works (or even if it will solve these > kinds of problems) so will say no more... > > E > > > > > On Aug 26, 2:10 am, Eric Abrahamsen <[EMAIL PROTECTED]> wrote: > >> On Aug 25, 2008, at 3:11 PM,MrJogowrote: > > >>> How do I create a custom manager for many-to-many traversal? An > >>> example will illustrate what I want to do better. Suppose I have the > >>> following models.py: > > >>> class Book(models.Model): > >>> title = models.CharField(max_length=100) > > >>> class Author(models.Model): > >>> books = models.ForeignKey(Book) > >>> name = models.CharField(max_length=100) > >>> is_alive = models.BooleanField() > > >>> This is a many-to-many relationship: a book can have multiple > >>> authors > >>> and an author can have written multiple books. > > >>> If I have a book object my_book, and I want to get all the > >>> authors, I > >>> do my_book.author_set.all(). How can I set up a custom manager to > >>> only > >>> get living authors of that book, so I could do something like > >>> my_book.livingauthor_set.all()? > > >> Custom managers are usually used for table-level functionality, ie > >> you'd make a manager method that filters or acts on a *group* of > >> books, not one single book. The usual thing to do when you want > >> something from a single object is to put a custom method on Book, > >> which returns living authors, like: > > >> def living_authors(self): > >> return self.author_set.filter(is_alive=True) > > >> What I don't know is if there's any way to keep this from making > >> another db hit every time you call some_book.living_authors(). > >> QuerySets can't be further filtered once they've been evaluated, so > >> using select_related() on the original Book QuerySet might not > >> help... > > >> Yours, > >> Eric > > >>> Thanks --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
