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
20535	Unnecessary join created for intermediate table between two M2M tables	German M. Bravo	nobody	"In this example:
{{{#!python
from django.db import models

class Person(models.Model):
    name = models.CharField(max_length=128)
    friends = models.ManyToManyField('self', symmetrical=False, through='Relationship')

    def __unicode__(self):
        return self.name

class Relationship(models.Model):
    from_person = models.ForeignKey(Person, related_name='idols')
    to_person = models.ForeignKey(Person, related_name='followers')
}}}

Example data for a test:
{{{#!python
ringo, _ = Person.objects.get_or_create(name=""Ringo Starr"")
paul, _ = Person.objects.get_or_create(name=""Paul McCartney"")
george, _ = Person.objects.get_or_create(name=""George Harrison"")
john, _ = Person.objects.get_or_create(name=""John Lennon"")
yoko, _ = Person.objects.get_or_create(name=""Yoko Ono"")

Relationship.objects.get_or_create(from_person=ringo, to_person=paul)
Relationship.objects.get_or_create(from_person=george, to_person=john)
Relationship.objects.get_or_create(from_person=paul, to_person=ringo)
Relationship.objects.get_or_create(from_person=paul, to_person=george)
Relationship.objects.get_or_create(from_person=paul, to_person=john)
Relationship.objects.get_or_create(from_person=john, to_person=paul)
Relationship.objects.get_or_create(from_person=john, to_person=yoko)
Relationship.objects.get_or_create(from_person=yoko, to_person=john)
}}}

Then doing (get idols of the idols of paul):
{{{#!python
    print Relationship.objects.filter(from_person__followers__from_person=paul).query
}}}

This produces a join between `Relationship` and `Person`, then to `Relationship`, then to `Person`. The final join (to `Person`) gets trimmed by `trim_joins()` after the `setup_joins()` in `Query.add_filter()`. However, the unneeded join with `Person` (between the two joined `Relationship`) is not removed. This produces something like:
{{{#!sql
SELECT ... FROM relationship INNER JOIN person ON (relationship.from_person_id = person.id) INNER JOIN relationship T3 ON (person.id = T3.to_person_id) WHERE T3.from_person_id = 2;
}}}

When in fact all that would be needed is:
{{{#!sql
SELECT ... FROM relationship INNER JOIN relationship T3 ON (relationship.from_person_id = T3.to_person_id) WHERE T3.from_person_id = 2
}}}
"	Bug	new	Database layer (models, ORM)	dev	Normal			Zach	Accepted	1	0	0	1	0	0