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)
>
My opinion: lets go forward with this API. Alternate is to add a dedicated
method for custom queryset prefetches, so that prefetch_related() isn't
overloaded. I am not sure if that is needed, so lets pass...
I'd say that prefetching from outer query to custom inner query should
never be allowed. This solves neatly issues like trying to prefetch into
.values() query.
Other opinions?
- Anssi
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