On Aug 25, 2008, at 3:11 PM, MrJogo wrote:
>
> 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
> >
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