On Thu, Jul 30, 2015 at 4:08 PM, Rich Lewis <[email protected]> wrote: > Dear All, > > I'm new to the Django ORM, and quite new to ORMs in general. I have two > models (lets call them A and B) between which I have an interesting mapping. > There are precisely 2 B instances associated with each A instance. Each A > instance can have many B instances. The order of Bs are important for As. > > I want to do something like this: > > class A(models.Model): > b_1 = models.ForeignKey(B) > b_2 = models.ForeignKey(B)
This would fail here, when you have more than one foreign key to the same model, you must supply a related_name argument on one of them, which is used for the inverse relation on the related object. Which leads us in to... > > class B(models.Model): > pass > > Such that i can do: > >>>> b1, b2, b3 = B(), B(), B() >>>> a1, a2 = A(b_1=b1, b_2=b2), A(b_1=b2, b_2=b3) >>>> b2.as > [<A ... >, <A ... >] #(order doesn't matter) Typically, B instances would have an attribute named 'a_set' (the lower case model name of the model it is related to, with "_set" appended). When you have multiple relationships with the same model, as I mentioned you must supply your own related_name arguments. In which case, with the models you mentioned, you could then do something like this: >>> b2.a1_set.all() | b2.a2_set.all() > [<A ..>, <A ..>] > > I expect I could eventually do something a bit hacky that would work, but > what would be the best way to handle this? I would say the above is the hacky way. You could have the same B assigned to b_1 and b_2 and would have duplicates in the set, and no way to specify DB constraints to limit it. Given that you anticipate being able to retrieve all the A's for a particular B, regardless of how they are related, I would model it as a M2M with a through table holding any additional information about the relationship, using the through table to add DB level constraints. Cheers Tom -- You received this message because you are subscribed to the Google Groups "Django users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/django-users. To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/CAFHbX1%2B7-KieRLGT_hG-UDC6zeyg%2Bb8d%3D7XtEK4EEM8nFvzFng%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
