Add example of Prefetch object usage to docs

[Ram Rachum]

Example needed here to clarify: ​https://docs.djangoproject.com/en/1.7/ref/models/queries/#django.db.models.Prefetch

[Collin Anderson]

agreed

[Collin Anderson]

Here's a snippet from my code:

posts = Post.objects.filter(post_type='post', post_status='publish').exclude(id=self.id).order_by('-post_date')
self.categories.prefetch_related(Prefetch('post_set', queryset=posts, to_attr='posts'))

[sww]

Here's my stab at an example:

>>> Question.objects.prefetch_related(Prefetch('choice_set')).get().choice_set.all()
[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]

>>> voted_choices = Choice.objects.filter(votes__gt=0)
>>> voted_choices
[<Choice: The sky>]

>>> Question.objects.prefetch_related(Prefetch('choice_set', queryset=voted_choices)).get().choice_set.all()
[<Choice: The sky>]

# Providing the to_attr value will preserve the choice_set results.
>>> Question.objects.prefetch_related(Prefetch('choice_set', queryset=voted_choices, to_attr='voted_choices')).get().voted_choices
[<Choice: The sky>]
>>> Question.objects.prefetch_related(Prefetch('choice_set', queryset=voted_choices, to_attr='voted_choices')).get().choice_set.all()
[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]

Feedback?

[Collin Anderson]

Seems fine to me, though shorter would be better in my opinion. Want to make a pull request?

[Simon Charette]

The pull request LGTM but a final review from a native speaker can't hurt :)

[Simon Charette <charette.s@…>]

In a3e89f13:

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

[Simon Charette <charette.s@…>]

In e0e2df41:

[1.8.x] Fixed #24414 -- Added examples of Prefetch object usage to the docs.

Backport of a3e89f13dfb1f22a26ead8b06b37695598a4421a from master