[Django] #23805: query `first` method clears cached queryset for prefetch_related
Django
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Reporter: alecklandgraf | Owner: nobody
Type: Uncategorized | Status: new
Component: Database layer (models, ORM) | Version: master
Severity: Normal | Keywords: first query
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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When a cached queryset is iterated through with `prefetch_related`, if
`.first()` is called during the iteration, a new DB query is fired off.
This is potentially due to the LIMIT 1 `[:1]` or the `order_by` added to
the queryset with the `first` method.
https://github.com/django/django/blob/master/django/db/models/query.py#L518
ex.
{{{
from django.db import connection
connection.queries = []
pizzas = Pizza.objects.all().prefetch_related('toppings')
print len(connection.queries) # 2
for pizza in pizzas:
first_topping = pizza.toppings.first()
print len(connection.queries) # 2 + number of pizzas
}}}
This fixes in my code at least:
{{{
from django.db import connection
connection.queries = []
pizzas = Pizza.objects.all().prefetch_related('toppings')
print len(connection.queries) # 2
for pizza in pizzas:
try:
first_topping = pizza.toppings.all()[0]
except IndexError:
first_topping = None
print len(connection.queries) # 2
}}}
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Ticket URL: <https://code.djangoproject.com/ticket/23805>
Django <https://code.djangoproject.com/>
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Django
Reporter: alecklandgraf | Owner: nobody
Type: Uncategorized | Status: new
(models, ORM) | Resolution:
Severity: Normal | Triage Stage:
Keywords: first query | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
Changes (by tchaumeny):
* needs_better_patch: => 0
* needs_tests: => 0
* needs_docs: => 0
Comment:
In the first code example, `first()` is used on an unordered queryset. As
`first()` is intended to be deterministic, it isn't surprising that it
tries to add an order condition (which default to `order_by('pk')`.
Your second code example might work, but it isn't deterministic either as
there is no guarantee on the ordering of `pizza.toppings.all()`.
Do you need an arbitrary element or the first one ? If you need the first
one, there should be some notion of ordering involved, otherwise your
second solution is fine.
Also, you might wanna have a look at
https://docs.djangoproject.com/en/1.7/ref/models/queries/#django.db.models.Prefetch
(I've never used it myself but it should allow more complex
prefetch_related queryset, including ORDER clause as described here
https://docs.djangoproject.com/en/1.7/ref/models/querysets/#prefetch-
related).
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Ticket URL: <https://code.djangoproject.com/ticket/23805#comment:1>
Django
Reporter: alecklandgraf | Owner: nobody
Type: Uncategorized | Status: closed
Component: Database layer | Version: master
Severity: Normal | Triage Stage:
Keywords: first query | Unreviewed
Has patch: 0 | Needs documentation: 0
Easy pickings: 0 | UI/UX: 0
Changes (by timgraham):
* status: new => closed
* resolution: => invalid
Comment:
tchaumeny has explained the issue and I don't see any bug here.
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Ticket URL: <https://code.djangoproject.com/ticket/23805#comment:2>