Re: Combining queries? A "join" in Python?

I looks like my attempt to simplify and abstract the problem just made
it harder to help me: my apologies for that. I was trying to combined
two different problems rather than ask two questions. Thanks for
helping despite that.

I think I have a solution for most of my problems, as far as getting
the queries working. I might be back with questions about
optimisation, but I can live with a little inefficiency in the query I
need to get working first.

On Aug 28, 4:33 am, Matteius <matte...@gmail.com> wrote:
> Since you are combining two sets of different objects you'll want
> Gelonida's response.  Additionally, Use Q to create more logically
> advanced queries.  I find your language difficult to gather what query
> you are looking for exactly, but here is an example of what I think
> you might mean:
>
> from django.db.models import Q
>
> the_Ds = D.objects.all().filter(b=B)
> the_Es = E.objects.all().filter(c=C)
>
> combined = the_Ds | the_Es
>
> # Other Example (an & requires both constraints to be met, and an Or
> will include the set of all matches.
> the_As = A.objects.all().filter(Q(constraint1) & Q(constraint2))
> the_Bs = B.objects.all().filter(Q(constraint1) | Q(constraint2))
>
> -Matteius
>
> Don't overlook how powerful this is because Django has Manager classes
> so you can make your constraints refer to both local properties and
> foreign key constraints as well as properties within the foreign key
> lookups.  You may find that you wish to edit your schema relationship,
> perhaps by pointing backwards or reversing the relationship.  It is
> hard to say with the categorical A, B, C, D example, but hope this
> helps and Good Luck!
>
> On Aug 27, 3:47 pm, Gelonida N <gelon...@gmail.com> wrote:
>
>
>
>
>
>
>
> > On 08/27/2011 11:39 AM, graeme wrote:
>
> > > I have a some models related link this:
>
> > > A has a foreign key on B which has a foreign key on C
>
> > > I also have D with a foreign key on B, and E with a foreign key of C
>
> > > If I do A.filter(**args).select_related() I will get all the As, Bs,
> > > and Cs I want.
>
> > > How do I get the Ds with a foreign key on a B in my queryset
> > > (preferably filtered) and all Es with a foreign key on C (also
> > > preferably filtered)
>
> > > The best I have been able to come up with is to query for all the  the
> > > Ds and Es I want, and combine them with the Bs and Cs in the view.
>
> > > I have a similar problem with another site, except that there not
> > > every B I want has an A with a foreignkey pointing to it, so I cannot
> > > just do select_related on A.
>
> > > I am not worried about doing an extra query or two per page. What I
> > > want to avoid is doing an extra query for each object in a lengthy
> > > list.
>
> > You can 'or' two query sets with the '|' operator
>
> > so do
> > queryset1 = Mymodel.objects.all().filter(...)
> > qyeryset2 = Mymodel.objects.all().filter(...)
>
> > combined = queryset1 | queryset2

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