On Tue, 2009-02-24 at 07:13 -0800, jgen...@jimmy.harvard.edu wrote:
> Hi there ...
>
> Suppose I have three Models:
>
> Model A
>
> Model B, has a M2M with Model A
>
> Model C, has M2Ms with both Model A and Model B
>
> What I'm trying to do is to construct a query which will answer the
> following question: "For a given A, what Cs does it have a
> relationship with, either through it's direct M2M or indirectly via
> Model B's M2M with Model C".
>
> I can do this piecemeal by getting the results from the A-C M2M, and
> then for each A-B pair get that B's results, and then unioning
> everything, but it'd be nice (and presumably The Right Way) to get it
> all in one command. It's this step that's confounding me. I'm
> guessing it requires using Q, but I still can't quite figure out the
> right magic to make this work.
Let's assume the models are described like this:
class A(models.Model):
pass
class B(models.Model):
to_a = models.ManyToManyField(A)
class C(models.Model):
to_a = models.ManyToManyField(A)
to_b = mdoels.ManyToManyField(B)
and suppose "obj_a" is your given A instance. Then I suspect this is the
type of queryset you're after:
C.objects.filter(Q(to_a=obj_a) | Q(to_b__to_a=obj_a))
Regards,
Malcolm
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