Added support for modifying the effect of `DISTINCT` clauses so they

only consider some fields (PostgreSQL only).

For this, the ``distinct()`` QuerySet method now accepts an optional
list of model fields names and generates ``DISTINCT ON`` clauses on
these cases. Thanks Jeffrey Gelens and Anssi Kääriäinen for their work.

Fixes #6422.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17244 bcc190cf-cafb-0310-a4f2-bffc1f526a37

tests/regressiontests/queries/models.py

@@ -209,6 +209,9 @@ class Celebrity(models.Model):
     name = models.CharField("Name", max_length=20)
     greatest_fan = models.ForeignKey("Fan", null=True, unique=True)
 
+    def __unicode__(self):
+        return self.name
+
 class TvChef(Celebrity):
     pass
 
@@ -343,4 +346,3 @@ class OneToOneCategory(models.Model):
 
     def __unicode__(self):
         return "one2one " + self.new_name
-