#29081: Clarify QuerySet.select_related() example regarding "hitting the
database"
--------------------------------------+------------------------------------
Reporter: Дилян Палаузов | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Documentation | Version: 2.0
Severity: Normal | Resolution:
Keywords: select_related | Triage Stage: Accepted
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by Tim Graham):
* has_patch: 0 => 1
* stage: Unreviewed => Accepted
Old description:
> https://docs.djangoproject.com/en/2.0/ref/models/querysets/#select-
> related says:
>
> > … then a call to Book.objects.select_related('author_
> _hometown').get(id=4) will cache the related Person and the related City:
> {{{
> b = Book.objects.select_related('author_ _hometown').get(id=4)
> p = b.author # Doesn't hit the database.
> c = p.hometown # Doesn't hit the database.
>
> b = Book.objects.get(id=4) # No select_related() in this example.
> p = b.author # Hits the database.
> c = p.hometown # Hits the database.
> }}}
>
> This leaves the impression, that the first snipped does not hit the
> database and the second snippet hits twice the database.
>
> I propose:
> * wring after {{{b = Book.objects.select_related('author_
> _hometown').get(id=4)}}} the comment # Hits the database with INNER JOIN.
> * changing {{{b = Book.objects.get(id=4) # No select_related() in this
> example.}}} to {{{b = Book.objects.get(id=4) # No select_related() in
> this example, hits the database.}}}
New description:
[[Image()]]https://docs.djangoproject.com/en/2.0/ref/models/querysets
/#select-related says:
> … then a call to Book.objects.select_related('author_
_hometown').get(id=4) will cache the related Person and the related City:
{{{
b = Book.objects.select_related('author_ _hometown').get(id=4)
p = b.author # Doesn't hit the database.
c = p.hometown # Doesn't hit the database.
b = Book.objects.get(id=4) # No select_related() in this example.
p = b.author # Hits the database.
c = p.hometown # Hits the database.
}}}
This leaves the impression, that the first snipped does not hit the
database and the second snippet hits twice the database.
I propose:
* wring after {{{b = Book.objects.select_related('author_
_hometown').get(id=4)}}} the comment # Hits the database with INNER JOIN.
* changing {{{b = Book.objects.get(id=4) # No select_related() in this
example.}}} to {{{b = Book.objects.get(id=4) # No select_related() in this
example, hits the database.}}}
--
Comment:
[https://github.com/django/django/pull/9629 PR]
--
Ticket URL: <https://code.djangoproject.com/ticket/29081#comment:2>
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