id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
28425	Prefetch can execute an UPDATE statement if a certain to_attr is given	kirillnedelev	nobody	"Following a django tutorial I've created polls application.
The only thing that was changed was a ForeignKey between Question and Choice models, null=True was added.

{{{
from django.db import models

class Question(models.Model):
    question_text = models.CharField(max_length=200)
    pub_date = models.DateTimeField('date published')


class Choice(models.Model):
    question = models.ForeignKey(Question, blank=True,null=True)
    choice_text = models.CharField(max_length=200)
    votes = models.IntegerField(default=0)
}}}

From the shell I've created one Question and one Choice:
{{{
from polls.models import Question, Choice
from django.utils import timezone

q = Question(question_text=""Question"", pub_date=timezone.now())
q.save()

q.choice_set.create(choice_text='Choice', votes=0)
}}}

After that I've executed:
{{{
from django.db.models import Prefetch

questions = (Question.objects
    .all()
    .prefetch_related(
        Prefetch('choice_set',
            queryset=(Choice.objects.filter(votes=5)),
            to_attr='choice_set'
        )
    )
)

print questions
}}}

And here are the queries that were executed:
{{{
(0.003) QUERY = u'SELECT ""polls_question"".""id"", ""polls_question"".""question_text"", ""polls_question"".""pub_date"" FROM ""polls_question"" LIMIT 21' - PARAMS = (); args=()
(0.000) QUERY = u'SELECT ""polls_choice"".""id"", ""polls_choice"".""question_id"", ""polls_choice"".""choice_text"", ""polls_choice"".""votes"" FROM ""polls_choice"" WHERE (""polls_choice"".""votes"" = %s AND ""polls_choice"".""question_id"" IN (%s))' - PARAMS = (5, 1); args=(5, 1)
(0.000) QUERY = u'BEGIN' - PARAMS = (); args=None
(0.001) QUERY = u'UPDATE ""polls_choice"" SET ""question_id"" = NULL WHERE ""polls_choice"".""question_id"" = %s' - PARAMS = (1,); args=(1,)
}}}

UPDATE query sets all question_id to NULL, which means that my question model doesn't have any choices now
{{{
q = Question(pk=1)
q.choice_set.all()
# []
}}}

I've traced the method where UPDATE statement was executed
**django/db/models/query.py**
{{{
def prefetch_one_level(instances, prefetcher, lookup, level):
...
    for obj in instances:
        instance_attr_val = instance_attr(obj)
        vals = rel_obj_cache.get(instance_attr_val, [])

        if single:
            val = vals[0] if vals else None
            to_attr = to_attr if as_attr else cache_name
            setattr(obj, to_attr, val)
        else:
            if as_attr:
->              setattr(obj, to_attr, vals)
            else:
                manager = getattr(obj, to_attr)
                if leaf and lookup.queryset is not None:
                    qs = manager._apply_rel_filters(lookup.queryset)
                else:
                    qs = manager.get_queryset()
                qs._result_cache = vals
                # We don't want the individual qs doing prefetch_related now,
                # since we have merged this into the current work.
                qs._prefetch_done = True
                obj._prefetched_objects_cache[cache_name] = qs
...
}}}

If I remove to_attr parameter from Prefetch, it works fine
{{{
questions = (Question.objects
    .all()
    .prefetch_related(
        Prefetch('choice_set',
            queryset=(Choice.objects.filter(votes=5))
            #,to_attr='choice_set'
        )
    )
)
}}}

But still it's risky behavior. "	Bug	closed	Database layer (models, ORM)	1.8	Normal	wontfix	risky behavior, db, queries	kiril@…	Accepted	0	0	0	0	0	0