On Aug 26, 2008, at 5:00 PM, MrJogo wrote:
> > 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/9be16ce8fc13643b/3fdadd9f82dcdf57 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 -~----------~----~----~----~------~----~------~--~---
