class Prefetch(lookup, queryset=None, to_attr=None) [source]
The Prefetch() object can be used to control the operation of prefetch_related().
The lookup argument describes the relations to follow and works the same as the string based lookups passed to prefetch_related(). For example:
>>> Question.objects.prefetch_related(Prefetch('choice_set')).get().choice_set.all()
<QuerySet [<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()
<QuerySet [<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 select_related() from the prefetched relation, hence reducing the number of queries even further:
>>> voted_choices = Choice.objects.filter(votes__gt=0)
>>> voted_choices
<QuerySet [<Choice: The sky>]>
>>> prefetch = Prefetch('choice_set', queryset=voted_choices)
>>> Question.objects.prefetch_related(prefetch).get().choice_set.all()
<QuerySet [<Choice: The sky>]>
The to_attr argument sets the result of the prefetch operation to a custom attribute:
>>> prefetch = Prefetch('choice_set', queryset=voted_choices, to_attr='voted_choices')
>>> Question.objects.prefetch_related(prefetch).get().voted_choices
<QuerySet [<Choice: The sky>]>
>>> Question.objects.prefetch_related(prefetch).get().choice_set.all()
<QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]>
Note
When using to_attr the prefetched result is stored in a list. This can provide a significant speed improvement over traditional prefetch_related calls which store the cached result within a QuerySet instance.
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