Support for 'DISTINCT ON' queries with QuerySet.distinct()

[Manfred Wassmann <manolo@…>]

The patch included modifies django/db/models/query.py so that the distinct method of the QuerySet object optionally takes variable number of field names and, if given, modifies the SQL statements generated so that 'DISTINCT ON (field,...)' is used. The incentive is to allow things like described in the following example:

class Example(models.Model):
  name = models.TextField()
  date = models. DateTimeField()
  other = models.XXX()
  ...

Now "Example.objects.all().distinct('name').order_by('name','-date')" will return the latest entry for each distinct name.
This can not otherwise be achieved unless resorting to plain SQL.

It should be noted that - at least in standard SQL and Postgres - if name allows NULL values, the query will return all entries for which name is NULL not only the latest as one might expect.

[Manfred Wassmann <manolo@…>]

Sorry, I didn't see this was already submitted before my latest changes.