#23805: query `first` method clears cached queryset for prefetch_related
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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>
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