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Fixed #24414 -- Added examples of Prefetch object usage to the docs.

This commit is contained in:
Sean Wang 2015-03-09 22:17:33 -07:00 committed by Simon Charette
parent 6b605be5fe
commit a3e89f13df

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@ -2862,15 +2862,35 @@ The ``Prefetch()`` object can be used to control the operation of
The ``lookup`` argument describes the relations to follow and works the same
as the string based lookups passed to
:meth:`~django.db.models.query.QuerySet.prefetch_related()`.
:meth:`~django.db.models.query.QuerySet.prefetch_related()`. For example:
>>> Question.objects.prefetch_related(Prefetch('choice_set')).get().choice_set.all()
[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]
# This will only execute two queries regardless of the number of Question
# and Choice objects.
>>> Question.objects.prefetch_related(Prefetch('choice_set')).all()
[<Question: Question object>]
The ``queryset`` argument supplies a base ``QuerySet`` for the given lookup.
This is useful to further filter down the prefetch operation, or to call
:meth:`~django.db.models.query.QuerySet.select_related()` from the prefetched
relation, hence reducing the number of queries even further.
relation, hence reducing the number of queries even further:
>>> voted_choices = Choice.objects.filter(votes__gt=0)
>>> voted_choices
[<Choice: The sky>]
>>> prefetch = Prefetch('choice_set', queryset=voted_choices)
>>> Question.objects.prefetch_related(prefetch).get().choice_set.all()
[<Choice: The sky>]
The ``to_attr`` argument sets the result of the prefetch operation to a custom
attribute.
attribute:
>>> prefetch = Prefetch('choice_set', queryset=voted_choices, to_attr='voted_choices')
>>> Question.objects.prefetch_related(prefetch).get().voted_choices
[<Choice: The sky>]
>>> Question.objects.prefetch_related(prefetch).get().choice_set.all()
[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]
.. note::