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
19843	ORM generate dupes when filtering twice on related objects	Artem Skoretskiy	nobody	"When I apply to a QuerySet a filter for related model twice -- I get dupes.

Let's say we have classes:
{{{
from django.db import models

class Film(models.Model):
    pass

class Person(models.Model):
    pass

class PersonInFilm(models.Model):
    film = models.ForeignKey(Film)
    person = models.ForeignKey(Person)
    title = models.CharField(max_length=50)
}}}

Then we try to find out needed PersonInFilm -- multiple filters (e.g. using Managers):
{{{
qs_wrong = Person.objects.filter(personinfilm__film=1).filter(personinfilm__role=1)
print ""%s rows\n%s"" % (qs_wrong.count(), qs_wrong.values('id').query)
}}}
This will result in:
{{{
15 rows
SELECT `persons_person`.`id` FROM `persons_person`
INNER JOIN `persons_personinfilm` ON (`persons_person`.`id` = `persons_personinfilm`.`person_id`)
INNER JOIN `persons_personinfilm` T4 ON (`persons_person`.`id` = T4.`person_id`)
WHERE (`persons_personinfilm`.`film_id` = 1  AND T4.`role_id` = 1 )
}}}

Then we try to find out needed PersonInFilm -- single filter:
{{{
qs_correct = Person.objects.filter(personinfilm__film=1, personinfilm__role=1)
print ""%s rows\n%s"" % (qs_correct.count(), qs_correct.values('id').query)
}}}
This will result in:
{{{
1 rows
SELECT `persons_person`.`id` FROM `persons_person`
INNER JOIN `persons_personinfilm` ON (`persons_person`.`id` = `persons_personinfilm`.`person_id`)
WHERE (`persons_personinfilm`.`role_id` = 1  AND `persons_personinfilm`.`film_id` = 1 )
}}}

Please keep in mind that often it is impossible to put filter in one command -- e.g. when you use custom Manager."	Bug	closed	Database layer (models, ORM)	1.4	Normal	invalid			Unreviewed	0	0	0	0	0	0