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Code Issues 32 Releases Wiki Activity

[1.7.x] Fixed #23732 -- Corrected and enhanced select_related() docs.

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Thanks Daniele Procida for the report and review.

Backport of e958c760f9 from master
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Tim Graham 2014-10-31 15:38:46 -04:00
parent c2cad66e47
commit e876c7f1ee
1 changed files with 20 additions and 2 deletions
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22
docs/ref/models/querysets.txt
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@ -754,6 +754,24 @@ And here's ``select_related`` lookup::
# in the previous query. # in the previous query.
b = e.blog b = e.blog
You can use ``select_related()`` with any queryset of objects::
from django.utils import timezone
# Find all the blogs with entries scheduled to be published in the future.
blogs = set()
for e in Entry.objects.filter(pub_date__gt=timezone.now()).select_related('blog'):
# Without select_related(), this would make a database query for each
# loop iteration in order to fetch the related blog for each entry.
blogs.add(e.blog)
The order of ``filter()`` and ``select_related()`` chaining isn't important.
These querysets are equivalent::
Entry.objects.filter(pub_date__gt=timezone.now()).selected_related('blog')
Entry.objects.selected_related('blog').filter(pub_date__gt=timezone.now())
You can follow foreign keys in a similar way to querying them. If you have the You can follow foreign keys in a similar way to querying them. If you have the
following models:: following models::
@ -771,10 +789,10 @@ following models::
# ... # ...
author = models.ForeignKey(Person) author = models.ForeignKey(Person)
... then a call to ``Book.objects.select_related('person__city').get(id=4)`` ... then a call to ``Book.objects.select_related('author__hometown').get(id=4)``
will cache the related ``Person`` *and* the related ``City``:: will cache the related ``Person`` *and* the related ``City``::
b = Book.objects.select_related('person__city').get(id=4) b = Book.objects.select_related('author__hometown').get(id=4)
p = b.author # Doesn't hit the database. p = b.author # Doesn't hit the database.
c = p.hometown # Doesn't hit the database. c = p.hometown # Doesn't hit the database.