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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