#26523: Unable to prefetch the implicit one to one relationship
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Reporter: czpython | Owner: nobody
Type: Bug | Status: new
Component: Database layer (models, ORM) | Version: 1.8
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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Given the following models:
{{{
class Fruit(models.Model):
weight = models.PositiveIntegerField(default=100)
class Apple(Fruit):
farmer = models.CharField(max_length=200, blank=True)
class Kiwi(models.Model):
fruit = models.OneToOneField(Fruit)
farmer = models.CharField(max_length=200, blank=True)
}}}
I would like to prefetch the fruit object related to an apple or kiwi.
So I ran the following:
{{{
In [2]: kiwi = Kiwi.objects.select_related('fruit').latest('id')
QUERY = u'SELECT "fruits_kiwi"."id", "fruits_kiwi"."fruit_id",
"fruits_kiwi"."farmer", "fruits_fruit"."id", "fruits_fruit"."weight" FROM
"fruits_kiwi" INNER JOIN "fruits_fruit" ON ( "fruits_kiwi"."fruit_id" =
"fruits_fruit"."id" ) ORDER BY "fruits_kiwi"."id" DESC LIMIT 1' - PARAMS =
() [0.84ms]
In [3]: kiwi.fruit
Out[3]: <Fruit: Fruit #00021>
}}}
As you can see, calling kiwi.fruit does not result in any extra queries.
Now I tried with Apple:
{{{
In [4]: apple = Apple.objects.select_related('fruit_ptr').latest('id')
QUERY = u'SELECT "fruits_fruit"."id", "fruits_fruit"."weight",
"fruits_apple"."fruit_ptr_id", "fruits_apple"."farmer" FROM "fruits_apple"
INNER JOIN "fruits_fruit" ON ( "fruits_apple"."fruit_ptr_id" =
"fruits_fruit"."id" ) ORDER BY "fruits_apple"."fruit_ptr_id" DESC LIMIT 1'
- PARAMS = () [0.31ms]
In [5]: apple.fruit_ptr
QUERY = u'SELECT "fruits_fruit"."id", "fruits_fruit"."weight" FROM
"fruits_fruit" WHERE "fruits_fruit"."id" = %s' - PARAMS = (16,) [0.35ms]
Out[5]: <Fruit: Fruit #00016>
}}}
Calling apple.fruit_ptr results in a query to the database even though
it's set to prefetch the related fruit.
I've traced the bug to
https://github.com/django/django/blob/stable/1.8.x/django/db/models/query_utils.py#L186
--
Ticket URL: <https://code.djangoproject.com/ticket/26523>
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