[django] Select DISTINCT individual columns in django?

I'm curious if there's any way to do a query in Django that's not a "SELECT * FROM..." underneath. I'm trying to do a "SELECT DISTINCT columnName FROM ..." instead.

Specifically I have a model that looks like:

class ProductOrder(models.Model):
   Product  = models.CharField(max_length=20, promary_key=True)
   Category = models.CharField(max_length=30)
   Rank = models.IntegerField()

where the Rank is a rank within a Category. I'd like to be able to iterate over all the Categories doing some operation on each rank within that category.

I'd like to first get a list of all the categories in the system and then query for all products in that category and repeat until every category is processed.

I'd rather avoid raw SQL, but if I have to go there, that'd be fine. Though I've never coded raw SQL in Django/Python before.

The other answers are fine, but this is a little cleaner, in that it only gives the values like you would get from a DISTINCT query, without any cruft from Django.

>>> set(ProductOrder.objects.values_list('category', flat=True))
{u'category1', u'category2', u'category3', u'category4'}

or

>>> list(set(ProductOrder.objects.values_list('category', flat=True)))
[u'category1', u'category2', u'category3', u'category4']

And, it works without PostgreSQL.

This is less efficient than using a .distinct(), presuming that DISTINCT in your database is faster than a python set, but it's great for noodling around the shell.

It's quite simple actually if you're using PostgreSQL, just use distinct(columns) (documentation).

Productorder.objects.all().distinct('category')

Note that this feature has been included in Django since 1.4

User order by with that field, and then do distinct.

ProductOrder.objects.order_by('category').values_list('category', flat=True).distinct()