#17001: Allow usage of custom querysets in prefetch_related
-------------------------------------+-------------------------------------
Reporter: akaariai | Owner: loic84
Type: New feature | Status: assigned
Component: Database layer | Version: master
(models, ORM) | Resolution:
Severity: Normal | Triage Stage: Accepted
Keywords: prefetch_related | Needs documentation: 1
Has patch: 1 | Patch needs improvement: 1
Needs tests: 1 | UI/UX: 0
Easy pickings: 0 |
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Comment (by loic84):
WIP: https://github.com/loic/django/compare/prefetch_related.
It's rather experimental and still needs tons of work with tests and docs,
but if anyone want to give it a try, that would help.
`Prefetch` takes two parameters: `lookup` and an optional `queryset`.
1. `Manager` backed lookups with `*args`:
{{{#!python
# Traditional string lookup:
Restaurant.objects.prefetch_related('pizzas__toppings')
# Equivalent with Prefetch:
Restaurant.objects.prefetch_related(Prefetch('pizzas__toppings'))
# Lookup with custom queryset:
Restaurant.objects.prefetch_related(
Prefetch('pizzas__toppings',
queryset=Toppings.object.filter(veggie=True)))
}}}
2. `list` backed lookups with `**kwargs`:
{{{#!python
# Traditional lookup stored in a list (significantly faster, see
#20577 and link below)
Restaurant.objects.prefetch_related(pizza_list=Prefetch('pizzas'))
# With custom QuerySet.
Pizza.objects.prefetch_related(
toppings_list=Prefetch('toppings',
queryset=Toppings.object.filter(veggie=True))
# Multiple level using nested prefetch_related:
pizza_queryset = Pizza.objects.prefetch_related(
Prefetch('toppings',
queryset=Toppings.object.filter(veggie=True)))
Restaurant.objects.prefetch_related(
pizza_list=Prefetch('pizzas', queryset=pizza_queryset))
# Alternatively, multiple level using kwarg lookups (nested
prefetch_related actually rewrite to that):
Restaurant.objects.prefetch_related(
pizza_list=Prefetch('pizzas'),
pizza_list__toppings_list=Prefetch('pizza_list__toppings',
queryset=Toppings.object.filter(veggie=True))
}}}
3. Mixing both approaches:
{{{#!python
Restaurant.objects.prefetch_related(
'pizza_list__toppings_list'
'pizza_list': Prefetch('pizza'),
)
}}}
4. Flattening is **not** supported, it doesn't seem trivial to implement,
maybe for another ticket?
{{{#!python
Restaurant.objects.prefetch_related(
toppings_list=Prefetch('pizzas__toppings')
)
}}}
Additionally, here is a quick and dirty benchmark that compares lookups
backed by `list` vs `Manager`: https://gist.github.com/loic/7198193.
--
Ticket URL: <https://code.djangoproject.com/ticket/17001#comment:33>
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