#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 --------------------------------------+------------------------------------ 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> Django <https://code.djangoproject.com/> The Web framework for perfectionists with deadlines. -- You received this message because you are subscribed to the Google Groups "Django updates" group. To unsubscribe from this group and stop receiving emails from it, send an email to django-updates+unsubscr...@googlegroups.com. To post to this group, send email to django-updates@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/django-updates/072.92bb131621c63ca9eef8ff1e14c9e806%40djangoproject.com. For more options, visit https://groups.google.com/d/optout.