#22508: select_related on a foreign related object fails to load fields on
original
object
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Reporter: boxm | Owner: nobody
Type: Uncategorized | Status: new
Component: Database layer (models, ORM) | Version: 1.6
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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Consider these models:
{{{
class Show(models.Model):
pass
class Poll(models.Model):
event = models.ForeignKey('Event')
class Event(models.Model)
show = models.ForeignKey(Show)
}}}
The following code fails to load the poll.event.show attribute:
{{{
In [3]: event = Event.objects.all()[2]
In [4]: poll = event.poll_set.select_related('event',
'event__show').all()[0]
In [5]: hasattr(poll.event, '_show_cache')
Out[5]: False
}}}
For comparison, in 1.4 the same code produces:
{{{
In [12]: event = Event.objects.all()[2]
In [13]: poll = event.poll_set.select_related('event',
'event__show').all()[0]
In [14]: hasattr(poll.event, '_show_cache')
Out[14]: True
}}}
The bug appears to be in the known_objects code around line 247 in
db/models/query.py which looks like it will overwrite the poll.event
attribute that was just retrieved from the DB (via select_related) and
replaces it with the parent model, which in this case doesn't have the
show attribute loaded.
Since the ORM was explicitly instructed to load the Event model in the
call, it shouldn't then replace the poll.event attribute with the
previously-known parent model.
--
Ticket URL: <https://code.djangoproject.com/ticket/22508>
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