Getting Unique Foreign Keys in Django?

Question

Suppose my model looks like this:

class Farm(models.Model):
   name = ...

class Tree(models.Model):
   farm = models.ForeignKey(Farm)

...and I get a QuerySet of Tree objects. How do I determine what farms are represented in that QuerySet?

Answer 1

http://docs.djangoproject.com/en/dev/ref/models/querysets/#in

Farm.objects.filter(tree__in=TreeQuerySet)

Answer 2

There might be a better way to do it with the Django ORM and keep it lazy but you can get what you want with regular python (off the top of my head):

>>> set([ t.farm for t in qs ])