Just a small note, I've done a small prefetch framework (mind you, before
prefetch_related was implemented) here:
https://github.com/ionelmc/django-prefetch
It allows you to have a tailored prefetching process. It's a bit low level
but it supports all the scenarios you mentioned. If you think it's
interesting I can write the equivalent examples ...
On Friday, August 23, 2013 6:03:13 PM UTC+3, Marc Tamlyn wrote:
>
> Ticket #17001 concerns the ability to customise the querysets used when
> using `prefetch_related()`. This has a working implementation with a less
> than ideal API - see the patches on the ticket.
>
> As the API design seems to be the main issue here, Anssi and I bounced
> some ideas around on IRC, and I thought it best to propose our best
> approach here to gather opinions on whether the API is palatable.
>
> Firstly note that the existing API would continue to work as it does now:
> Publisher.objects.prefetch_related('authors', 'authors__books')
> You can then access publisher.authors.all() as before, each of which will
> allow you to access author.books.all()
>
> If you wanted to prefetch the authors ordered by their age rather than
> whatever the default is, you can do:
> Publisher.objects.prefetch_related('authors', to_attr='authors_by_age',
> queryset=Author.objects.order_by('age'))
> The authors are now available as publisher.authors_by_age - this is a list
> (not a qs) of the relevant Author objects.
>
> Only one custom field can be prefetched at once in this way. So if you
> wanted to get the books prefetched as well, you would need to do:
> Publisher.objects.prefetch_related('authors', to_attr='authors_by_age',
> queryset=Author.objects.order_by('age').prefetch_related('books'))
>
> An alternative API that may work for this case is:
> Publisher.objects.prefetch_related('authors', to_attr='authors_by_age',
> queryset=Author.objects.order_by('age')).prefetch_related('authors_by_age__books')
> i.e. the custom prefetch can be referenced in future chained prefetch
> calls.
>
> This API can be used to customise the second level only:
> Publisher.objects.prefetch_related('authors').prefetch_related('authors__books',
>
> to_attr='unpublished_books', queryset=Book.objects.filter(published=False))
>
> If you want to customise both levels though, you must use the first
> approach:
> author_qs = Author.objects.order_by('age').prefetch_related('books',
> to_attr='unpublished_books', queryset=Book.objects.filter(published=False))
> Publisher.objects.prefetch_related('authors', to_attr='authors_by_age',
> queryset=author_qs)
>
>
> Opinions are welcome!
>
> Marc
>
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