F.objects.select_related('g__n') looks a bit convoluted to me.
You can already do something like F.objects.select_related('g').only('g__n',
*[f.name for f in F._meta.get_fields()]) and write a small helper function
for the list comprehension to avoid typing.
Also I'd say it's generally a rare case that such an optimization is worth
it, so it's probably not useful to add ORM shortcut syntax for it.
On 26 January 2018 at 17:24, Дилян Палаузов <dpa-dja...@aegee.org> wrote:
> Hello,
>
> Provided:
>
> class M(models.Model):
> n = models.IntegerField()
> r = models.IntegerField()
> t = models.IntegerField()
> s = models.IntegerField()
> u = models.IntegerField()
>
> class F(models.Model):
> g = models.ForeignKey(M, on_delete=models.CASCADE)
> a = models.IntegerField()
> b = models.IntegerField()
> c = models.IntegerField()
> d = models.IntegerField()
> e = models.IntegerField()
>
>
> I want to obtain with a queryset all fields from F and M.n, but not the
> other fields of M like M.r, so that a single SELECT is generated, that
> doesn't pick unnecessary columns.
>
> F.objects.select_related('g').defer('g__r', 'g__t', 'g__s', 'g__u') does
> work, but I don't like it, as I want to say only M.n has to be obtained,
> and not the others.
>
> prefetch_related() is also suboptimal, as it creates two SQL queries
> instead of one. only() does not let me specify 'All fields of F'.
>
> So here neither only() nor defer() nor prefetch_related() are good.
>
> F.objects.select_related('g__n') would fit from logic, but does not work
> (django.core.exceptions.FieldError: Non-relational field given in
> select_related: 'n'. Choices are: (none))
>
> I propose teaching select_related() to accept the above expression.
>
> Confer https://code.djangoproject.com/ticket/29072 .
>
> Regards
> Дилян
>
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