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.
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. 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 -~----------~----~----~----~------~----~------~--~---
