Redundant joins generated when using reverse foreign key

[Andy Durdin]

A query using a reverse foreign key generates a left outer join to a related table and a second, redundant left outer join to the original table.

Tested in Django 1.1 and trunk r15147, and found to be an issue in both.

Using these models:

from django.db import models

class Original(models.Model):
    pass # Don't need anything more for this test

class Reference(models.Model):
    original = models.ForeignKey(Original, primary_key=True)

Background: the Reference model is used for storing and testing set membership; an Original instance is a member of the set iff its PK is in the Reference table.

Incorrect query was generated for this queryset:

Original.objects.filter(reference__pk__isnull=True)

Generated SQL:

SELECT `fkpk_original`.`id` FROM `fkpk_original`
LEFT OUTER JOIN `fkpk_reference` ON (`fkpk_original`.`id` = `fkpk_reference`.`original_id`)
LEFT OUTER JOIN `fkpk_original` T3 ON (`fkpk_reference`.`original_id` = T3.`id`)
WHERE T3.`id` IS NULL

Expected SQL:

SELECT `fkpk_original`.`id` FROM `fkpk_original`
LEFT OUTER JOIN `fkpk_reference` ON (`fkpk_original`.`id` = `fkpk_reference`.`original_id`)
WHERE `fkpk_reference`.`original_id` IS NULL

Notes

An incorrect query is also generated for the following queryset; however this seems to be covered by #14056:

Original.objects.filter(reference__pk__isnull=False)

[Russell Keith-Magee]

I'm not sure I see the connection with #14056, but this certainly appears to be a problem. I'm going to guess that it's the primary_key=True on a ForeignKey that is causing the confusion.

[Andy Durdin]

To clarify: I found incorrect queries when I was using the two variations on this queryset, ...isnull=True, and ...isnull=False. I presumed that they were related, since it was only a parameter to the queryset that I was changing. I found #14056 when searching to see if this was a reported issue, and found that its description and patch matched and fixed only the second of the two incorrect queries. That could be coincidental; I wasn't able to follow the query generation well enough to know.

[Russell Keith-Magee]

Not sure how this got assigned to me

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Tim Graham]

Seems to be fixed in Django 1.6.