id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
17001	Allow usage of custom querysets in prefetch_related	Anssi Kääriäinen	loic84	"#16973 introduced prefetch_related. I would like to extend it so that one can use custom querysets for the fetching. This also requires the ability to define the attribute name to use for the prefetched objects, as it is not valid to return filtered objects from related_manager.all(). There is also the possibility that one wants to fetch multiple objects of the same type with prefetch_related (young authors to one attribute, old authors to another).

The proposed API is following:
{{{
# The normal usage does not change
SomeModel.objects.prefetch_related('rel_model', 'rel_model__another_rel')
# You can pass R objects to prefetch_related
# The syntax for R is
# R(lookup_path, to_attr=None, qs=None)
# Thus:
SomeModel.objects.prefetch_related(R('rel_model'), R('rel_model__another_rel'))
# is equivalent to the first call.
# One can use to_attr alone:
SomeModel.objects.prefetch_related(R('rel_model', to_attr='foo_lst'), ...)
# custom queryset
SomeModel.objects.prefetch_related(
    R('rel_model', to_attr='foo_lst', qs=RelModel.objects.order_by('foo')),
    ...
)
# If qs is defined, to_attr must be defined also:
SomeModel.objects.prefetch_related(R('rel_model', qs=RelModel.objects.order_by('-pk')))
-> Error: Thou shalt not use qs without to_attr
# One can use the to_attr in nested calls:
qs = SomeModel.objects.prefetch_related(
   R('rel_model', qs=RelModel.objects.order_by('-pk'), to_attr='foo_lst'),
   R('foo_lst__another_model', qs=AnotModel.objects.order_by('-pk'), to_attr='anot_lst')
)
# This will result in every object O in foo_lst to have attribute anot_lst containing
# related objects to O, fetched from the above given qs.
# Going still deeper would need to use syntax:
qs = qs.prefetch_related(
   R('foo_lst__anot_lst__third_model', qs=ThirdModel.objects.order_by('-pk'), to_attr='bar_lst'),
)
}}}
"	New feature	closed	Database layer (models, ORM)	dev	Normal	fixed	prefetch_related	marc.tamlyn@… AndrewIngram k.sikora@… ethan.jucovy@… as@… mszamot@… jonatan.lundin@…	Accepted	1	0	0	0	0	0