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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