Opened 7 years ago

Last modified 7 years ago

#29081 closed Cleanup/optimization

Clarify QuerySet.select_related() example regarding "hitting the database" — at Version 2

Reported by: Дилян Палаузов Owned by: nobody
Component: Documentation Version: 2.0
Severity: Normal Keywords: select_related
Cc: Triage Stage: Accepted
Has patch: yes Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description (last modified by Tim Graham)

​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.

Change History (2)

comment:1 by Дилян Палаузов, 7 years ago

Description: modified (diff)

comment:2 by Tim Graham, 7 years ago

Description: modified (diff)
Has patch: set
Summary: select_related and hitting the database → Clarify QuerySet.select_related() example regarding "hitting the database"
Triage Stage: Unreviewed → Accepted

​PR

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