What you could do is create the intermediary table explicitly using the through keyword: http://docs.djangoproject.com/en/dev//topics/db/models/#extra-fields-on-many-to-many-relationships then you can do queries on the table.
Alex On Oct 1, 7:17 pm, "Dan W." <[EMAIL PROTECTED]> wrote: > This is my first time building an app with django and so far I've been > more than happy with it. However, I can't seem to figure out how to > sufficiently optimize queries for ManyToMany relationships. Here is a > model to illustrate my problem: > > class Book(models.Model): > name = models.CharField(max_length=255) > > class Reader(models.Model): > first_name = models.CharField(max_length=30) > last_name = models.CharField(max_length=30) > books_read = models.ManyToMany(Book, relatedName='read_by') > > Now I simply want to list every reader along with the books that > they've read. For example: > > Reader Books Read > ---------- > ---------------------------------------------------------- > Claire Smith Great Expectations > Jane Eyre > Wuthering Heights > Dan Smith Great Expectations > War and Peace > Adventures of Sherlock Holmes > Bob Smith Adventures of Sherlock Holmes > The Call of the Wild > War and Peace > .... .... > > Ideally select_related() would work with ManyToMany relationships as > well as ForeignKey relationships but I can see that this would take > the model query API to a whole other level of complixity so I can > understand why it hasn't been done yet. I'm used to ORM libraries like > Java's Hibernate that can fetch entire graphs of objects with a > minimum number of queries but I'm more than happy to do a little extra > work to avoid the complexity of something like Hibernate. > > However, I would hope that I could fetch the necessary data to > instantiate the object graph for any number of readers with only 2 > queries. The queries would look like: > readers = Reader.objects.filter(last_name='Smith'); > books = Book.objects.filter(read_by__last_name='Smith'); > > These two queries get me all the data I need but I still don't have > the connectivity information to know which readers have read which > books. In order to get this piece of info, it looks like I'll have to > drop into the native sql world and fetch the info directly from the > many-to-many table created by django. Is this what necessary to do the > database optimization or am I missing something? It's not so bad in > this simple case but as the graph of interesting objects gets bigger, > I'm afraid I may have to write a lot of this type of code. > > Thanks, > > --Dan --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
