On 26 marras, 21:36, tolomea <gordon.wrig...@gmail.com> wrote:
> The focus then is on how to let the developer specify what they want in the
> same query and what they want in a different query.
> Select_related currently adds same query stuff and prefetch_related
> adds separate query stuff. So what if we added a way for a select_related
> to be bolted onto a prefetch_related. A "base" argument to select_related
> seems like a reasonable way of doing this. The original example was:
>
> > Restaurant.objects.prefetch_related('best_pizza__toppings__topping_type')
>
> We could rewrite it as:
>
>
>
> Restaurant.objects.prefetch_related('best_pizza__toppings").select_related(base='best_pizza__toppings",
> "topping_type')
>
> The prefetch_related can be inferred from the base, so we could shorten it
> to:
>
> > Restaurant.objects.select_related(base='best_pizza__toppings",
>
> "topping_type')
>
> Each unique unique base produce a new db query, multiple select_relateds
> with the same base are done as one query.
> I think that's actually a complete system in itself, prefetch_related is
> just shorthand and can be implemented as:
>
> def prefetch_related(queryset, *args):
> for arg in args:
> queryset = queryset.select_related(base=arg)
> return queryset
>
> The only quirk comes from the original example again, what does this do?
>
> Restaurant.objects.prefetch_related('best_pizza__toppings__topping_type')
>
> Tired, but I think there is something to this line of thinking.
The above could work. I would maybe do it the other way, something
along lines: qs.prefetch_related('best_pizza__toppings__topping_type',
use_select_related=True). This would tell the prefetching to use
automatically select_related for any lookups encountered assuming it
is possible to select_related the data. For those lookups which are
multijoins prefetch would work as always. If you want to prefetch the
best_pizza, but select_related the topping_type then use
qs.prefetch_related('best_pizza__toppings').prefetch_related('best_pizza__toppings__topping_type',
use_select_related=True)
Custom queryset use would also be useful. Maybe you want to order the
toppings some other way than the default ordering. If a nice API
surfaces for this this would be a very nice addition.
- Anssi
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