Ordering by related attributes makes distinct() break

[Simon Law]

The problem is that sorting on an attribute of a related model causes distinct() to fail.

This is because the SELECT DISTINCT operates on the extra column required for the LEFT OUTER JOIN, which makes it impossible to group the desired results.

To reproduce the error, run this test case:

models.py

import django

from django.db import models


class Article(models.Model):
    title = models.CharField(max_length=128)

    def __unicode__(self):
        return self.title

class Comment(models.Model):
    article = models.ForeignKey(Article)
    comment = models.CharField(max_length=128)

    def __unicode__(self):
        return "%s on %s" % (self.comment, self.article.title)

tests.py

from django.test import TestCase
from django import db
from django.conf import settings

from .models import Article, Comment


class TestOrderDistinctBug(TestCase):
    def test(self):
        settings.DEBUG = True
        awesome = Article.objects.create(title="Awesome Title")
        different = Article.objects.create(title="Different Title")
        wonderfull = Article.objects.create(title="Bad Title")
        
        Comment.objects.create(article=awesome, comment="Agreed")
        Comment.objects.create(article=awesome, comment="Dummy")
        Comment.objects.create(article=different, comment="Yeah")
        Comment.objects.create(article=different, comment="Onoz ")
        Comment.objects.create(article=wonderfull, comment="First !")
        
        db.reset_queries()
        articles = Article.objects.order_by("comment__comment").distinct()

        print "articles=%s" % articles
        # articles=[<Article: Awesome Title>, <Article: Awesome Title>, <Article: Bad Title>, <Article: Different Title>, <Article: Different Title>]

        print "sql is : %s" % db.connection.queries[0]
        # sql is : {'time': '0.001', 
        # 'sql': u'SELECT DISTINCT `order_bug_article`.`id`, `order_bug_article`.`title`, `order_bug_comment`.`comment`
        # FROM `order_bug_article` LEFT OUTER JOIN `order_bug_comment` ON (
        #    `order_bug_article`.`id` = `order_bug_comment`.`article_id`
        # ) ORDER BY `order_bug_comment`.`comment` ASC LIMIT 21'}

        
        # count returns the right thing
        self.assertEquals(articles.count(), 3)
        # but the content of the query set is wrong
        self.assertEquals(len(articles), 3)

[Alex Gaynor]

This is documented: ​http://docs.djangoproject.com/en/dev/ref/models/querysets/#order-by-fields

[Simon Law]

Aw, it's documented, but it's still awful behaviour.

Would you guys take a patch that would fix this up?