1. No, the two will execute identically.
2. No, `group` is loaded by select_related, and `name` is a field on
group, so it's loaded when group is, unless you explicitly exclude it.
3. That's one query. However, if you're only grabbing the first result,
select_related isn't even necessary... just do:
name = p.groupregistration_set.all()[:1][0].group.name
However, note that BOTH lines will fail if the query returns no
results, due to the [0] index. I.e., if p.groupregistration_set.all()
returns 0 results, then the [0] is invalid.
4. I'm not sure I understand the question, could you expand?
_Nik
On 3/7/2013 10:35 AM, Fatih Tiryakioglu wrote:
> Thank you Nicolas,
>
> This works:
> reg = p.groupregistration_set.all().select_related('group')
> name = reg[0].group.name
>
> This also works:
> name = p.groupregistration_set.all().select_related('group')[0].group.name
>
>
> 1) Is there any difference? Which one is beter.
> 2) reg = p.groupregistration_set.all().select_related('group')
> name = reg[0].group.name
> does second line result in db hit???
> 3) name =
> p.groupregistration_set.all().select_related('group')[0].group.name
> this is 1 or 2 hit??
>
> and an optional 10 points question:
> 4) If I had two members of reg list, how to modify the code above?
>
>
> Thanks again..
>
>
> --
>
>
>
>
> --
>
> On Thursday, March 7, 2013 6:35:14 PM UTC+2, Nikolas Stevenson-Molnar
> wrote:
>
> The biggest hit is probably going to be the reg.group call. You can
> optimize that by using select_related (), which will take what is
> currently 1 + (number of group registrations for the user) queries
> and
> turn it into 2 queries:
>
> reg = p.groupregistration_set.all().select_related('group')
>
> More on select_related:
> https://docs.djangoproject.com/en/dev/ref/models/querysets/#select-related
>
> <https://docs.djangoproject.com/en/dev/ref/models/querysets/#select-related>
>
>
> _Nik
>
> On 3/6/2013 11:18 PM, Fatih Tiryakioglu wrote:
> > Hi all,
> >
> > Is there any shortcut (minimum db hit) for the following querysets:
> >
> > p = Person.objects.get(mail="pers...@ex.com <javascript:>")
> > p.password & form.password comparison and it is ok.
> >
> > #reversing to the person's groupregisteration, for example:
> > reg = p.groupregistration_set.all() #groupregistratin model is
> simply
> > person (foreign key) and group (foreign key)
> >
> > #evaluating group of reg list.
> > g = reg.group (for the every member of reg list)
> >
> > #and finally group name
> > n = g.name <http://g.name> (for the every member of g list)
> >
> > Which queryset or querysets hits the db, and how can I minimize it.
> >
> >
> >
> > Thanks.
> > --
> >
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