How to get average from set of objects in Django?

I have a simple rating system for a property. You give it a mark out of 5 (stars). The models are defined like this

def Property(models.Model)
    # stuff here

def Rating(models.Model)
    property = models.ForeignKey(Property)
    stars = models.IntegerField()

What I want to do is get a property, find all the Rating objects, collect them, then get the average 'stars' from them.

Any ideas how to do this?