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/modeltests/distinct_on_fields/models.py

@@ -0,0 +1,39 @@
+from django.db import models
+
+class Tag(models.Model):
+    name = models.CharField(max_length=10)
+    parent = models.ForeignKey('self', blank=True, null=True,
+            related_name='children')
+
+    class Meta:
+        ordering = ['name']
+
+    def __unicode__(self):
+        return self.name
+
+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 Fan(models.Model):
+    fan_of = models.ForeignKey(Celebrity)
+
+class Staff(models.Model):
+    id = models.IntegerField(primary_key=True)
+    name = models.CharField(max_length=50)
+    organisation = models.CharField(max_length=100)
+    tags = models.ManyToManyField(Tag, through='StaffTag')
+    coworkers = models.ManyToManyField('self')
+
+    def __unicode__(self):
+        return self.name
+
+class StaffTag(models.Model):
+    staff = models.ForeignKey(Staff)
+    tag = models.ForeignKey(Tag)
+
+    def __unicode__(self):
+        return u"%s -> %s" % (self.tag, self.staff)